01. Introduction to BGP
02. iBGP Peerings
03. Route Reflection
04. eBGP Peerings
05. Confederation
06. Update Source
07. Next Hop Processing
08. Advertising BGP Prefix Information
09. Summarization
10. BGP Default Routing
11. Best Path Selection
12. Best Path Selection Order
13. Manipulating Best Path Selection
14. Communities
15. Regular Expressions
16. Route Dampening
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Monday, June 29, 2009
Sunday, June 28, 2009
Friday, June 26, 2009
CCNA INTERVIEW Q&A
Q. What is a router?
A. A router is a device that connects more than one physical network, or segments of a network, using IP routing software. As packets reach the router, the router reads them and forwards them to their destination.
Q. Discuss wireless networking.
A. This is a network configured to use communication techniques such as infrared, cellular, or microwave, so that cable connections are not required.
Q. Discuss WAN (wide area network).
A. A WAN is extended over longer distances that a LAN (local area network). It can range from a few miles to across the world. TCP/IP is the primary WAN protocol and was developed to provide reliable, secure data transmissions over long distances.
Q. What is OSPF?
A. Open Shortest Path First is a routing protocol that supports the concept of a core area to which everything attaches.
Q. What is BGP?
A. Border Gateway Protocol is used for routing between networks on the Internet core, and it supports many advanced routing features.
Q. What is an autonomous system?
A. An autonomous system is a community of interest. Used in conjunction with routing protocols, it breaks up parts of the network into manageable chunks.
Q. What is dial on demand?
A. Dial on demand is a technology that only activates network connection when “interesting” packets are to be sent across the infrastructure.
Q. What mask would you use to supernet two class C addresses?
A. The subnet would be 255.255.254.0.
Q. What is VLANing?
A. Virtual LAN is used on large LANs to break up the network into smaller broadcast domains. This creates communities of interest. These communities can be based around organizational structures.
Q. What is CIDR?
A. Classless Internet domain routing is used in conjunction with classless routing protocols to summarize the Internet into smaller routing tables.
Q. What is VLSM?
A. Variable Length Subnet Mask is used to allocate the amount of address space required by the end network.
Q. What is a class D IP address?
A. Class D addresses are multicast addresses.
Q. What addresses do multicasts start with?
A. Multicasts start with the address 224.0.0.0.
Q. Which name resolution system is implemented with TCP/IP by default?
A. Although WINS is a name resolution that is implemented by TCP/IP by default, it only works on Windows-based networks. The only true name resolution system that almost every TCP/IP networks uses is DNS.
Q. You are the administrator of a 100-station Ethernet network. Your users are complaining of slow network speeds. What could you replace your hub with to increase your network throughput?
A. A switch would increase performance by making virtual, direct connections between sender and receiver. A bridge and router would actually decrease performance because these devices introduce latency into the communication.
Q. Which TCP/IP utility is most often used to test whether an IP host is up and functional?
A. The Ping utility is the most often used TCP/IP utility because it allows you to test individual hosts.
Q. Which utility can you use to find the MAC and TCP/IP address of your Windows NT or 2000 workstation?
A. The ipconfig utility is available for both these operating systems. It displays information like the MAC and TCP/IP address of your workstation as well as other TCP/IP configuration information.
Q. Which utility can you use to verify a packet’s path?
A. The tracert utility traces the route from the source IP host to the destination host.
Q. Which WAN technology uses digital signaling from sender to receiver?
A. The T-series of WAN connection (such as T1, T3, and so on) uses digital signaling from sending hardware to receiving hardware.
Q. You are setting up a workstation for remote access to the office. The office has a modem pool configured, and it is working correctly. The required results are that the workstation and modem bank must establish a connection and that the server at the office must authenticate the workstation. Optionally, the workstation and office must be able to communicate by using a single protocol, and the workstation must be able to access all network devices at the office. The proposed solution is to install a POTS telephone line, modem cable, and modem connected to the workstation. How would you configure the protocols to achieve the desired results?
A. This question tests your ability to configure protocols and select the best one to meet the connectivity requirements. The recommended protocol here would be TCP/IP since it can be used across the different access methods.
Q. Which remote access protocol can run over both serial and parallel connections?
A. Because PPP doesn’t contain a physical layer specification as part of the protocol, it can run over any kind of medium.
Q. What Microsoft TCP/IP protocol can be used over the Internet to create a secure, virtual network?
A. The Point-to-Point Tunneling Protocol (PPTP) allows you to create a secure, virtual connection between two points by tunneling one protocol inside another. Usually, a PPP connection is opened over a TCP/IP link.
Q. Which type of firewall checks for a current communication and the next packet needed?
A. A proxy provides firewall services by keeping track of all communications sessions and “prefetching” the next packets.
Q. Which type of security uses a file that identifies predefined IP addresses that are allowed to send data through a router?
A. Access Control List security uses a file (the ACL) that identifies which addresses can send data through a particular firewall or router.
A. A router is a device that connects more than one physical network, or segments of a network, using IP routing software. As packets reach the router, the router reads them and forwards them to their destination.
Q. Discuss wireless networking.
A. This is a network configured to use communication techniques such as infrared, cellular, or microwave, so that cable connections are not required.
Q. Discuss WAN (wide area network).
A. A WAN is extended over longer distances that a LAN (local area network). It can range from a few miles to across the world. TCP/IP is the primary WAN protocol and was developed to provide reliable, secure data transmissions over long distances.
Q. What is OSPF?
A. Open Shortest Path First is a routing protocol that supports the concept of a core area to which everything attaches.
Q. What is BGP?
A. Border Gateway Protocol is used for routing between networks on the Internet core, and it supports many advanced routing features.
Q. What is an autonomous system?
A. An autonomous system is a community of interest. Used in conjunction with routing protocols, it breaks up parts of the network into manageable chunks.
Q. What is dial on demand?
A. Dial on demand is a technology that only activates network connection when “interesting” packets are to be sent across the infrastructure.
Q. What mask would you use to supernet two class C addresses?
A. The subnet would be 255.255.254.0.
Q. What is VLANing?
A. Virtual LAN is used on large LANs to break up the network into smaller broadcast domains. This creates communities of interest. These communities can be based around organizational structures.
Q. What is CIDR?
A. Classless Internet domain routing is used in conjunction with classless routing protocols to summarize the Internet into smaller routing tables.
Q. What is VLSM?
A. Variable Length Subnet Mask is used to allocate the amount of address space required by the end network.
Q. What is a class D IP address?
A. Class D addresses are multicast addresses.
Q. What addresses do multicasts start with?
A. Multicasts start with the address 224.0.0.0.
Q. Which name resolution system is implemented with TCP/IP by default?
A. Although WINS is a name resolution that is implemented by TCP/IP by default, it only works on Windows-based networks. The only true name resolution system that almost every TCP/IP networks uses is DNS.
Q. You are the administrator of a 100-station Ethernet network. Your users are complaining of slow network speeds. What could you replace your hub with to increase your network throughput?
A. A switch would increase performance by making virtual, direct connections between sender and receiver. A bridge and router would actually decrease performance because these devices introduce latency into the communication.
Q. Which TCP/IP utility is most often used to test whether an IP host is up and functional?
A. The Ping utility is the most often used TCP/IP utility because it allows you to test individual hosts.
Q. Which utility can you use to find the MAC and TCP/IP address of your Windows NT or 2000 workstation?
A. The ipconfig utility is available for both these operating systems. It displays information like the MAC and TCP/IP address of your workstation as well as other TCP/IP configuration information.
Q. Which utility can you use to verify a packet’s path?
A. The tracert utility traces the route from the source IP host to the destination host.
Q. Which WAN technology uses digital signaling from sender to receiver?
A. The T-series of WAN connection (such as T1, T3, and so on) uses digital signaling from sending hardware to receiving hardware.
Q. You are setting up a workstation for remote access to the office. The office has a modem pool configured, and it is working correctly. The required results are that the workstation and modem bank must establish a connection and that the server at the office must authenticate the workstation. Optionally, the workstation and office must be able to communicate by using a single protocol, and the workstation must be able to access all network devices at the office. The proposed solution is to install a POTS telephone line, modem cable, and modem connected to the workstation. How would you configure the protocols to achieve the desired results?
A. This question tests your ability to configure protocols and select the best one to meet the connectivity requirements. The recommended protocol here would be TCP/IP since it can be used across the different access methods.
Q. Which remote access protocol can run over both serial and parallel connections?
A. Because PPP doesn’t contain a physical layer specification as part of the protocol, it can run over any kind of medium.
Q. What Microsoft TCP/IP protocol can be used over the Internet to create a secure, virtual network?
A. The Point-to-Point Tunneling Protocol (PPTP) allows you to create a secure, virtual connection between two points by tunneling one protocol inside another. Usually, a PPP connection is opened over a TCP/IP link.
Q. Which type of firewall checks for a current communication and the next packet needed?
A. A proxy provides firewall services by keeping track of all communications sessions and “prefetching” the next packets.
Q. Which type of security uses a file that identifies predefined IP addresses that are allowed to send data through a router?
A. Access Control List security uses a file (the ACL) that identifies which addresses can send data through a particular firewall or router.
Sunday, June 21, 2009
IP Access Control List (ACL)
Standard IP Access Control Lists
Filtering logic could be configured on any router and on any of its interfaces. Cisco IOS software applies the filtering logic of an ACL either as a packet enters an interface or as it exits the interface. In other words, IOS associates an ACL with an interface, and specifically for traffic either entering or exiting the interface. After you have chosen the router on which you want to place the access list, you must choose the interface on which to apply the access logic, as well as whether to apply the logic for inbound or outbound packets.
The key features of Cisco ACLs are:
. Packets can be filtered as they enter an interface, before the routing decision.
. Packets can be filtered before they exit an interface, after the routing decision.
. Deny is the term used in Cisco IOS software to imply that the packet will be filtered.
. Permit is the term used in Cisco IOS software to imply that the packet will not be filtered.
. The filtering logic is configured in the access list.
. If a packet does not match any of your access list statements, it is blocked.
Access lists have two major steps in their logic: matching, which determines whether it matches the access-list statement; and action, which can be either deny or permit. Deny means to discard the packet, and permit implies that the packet should be allowed. However, the logic that IOS uses with a multiple-entry ACL can be much more complex. Generally, the logic can be summarized as follows:
Step 1: The matching parameters of the access-list statement are compared to the packet.
Step 2: If a match is made, the action defined in this access-list statement (permit or deny) is performed.
Step 3: If a match is not made in Step 2, repeat Steps 1 and 2 using each successive statement in the ACL until a match is made.
Step 4: If no match is made with an entry in the access list, the deny action is performed.
Wildcard Masks
IOS IP ACLs match packets by looking at the IP, TCP, and UDP headers in the packet. Standard IP access lists can also examine only the source IP address. You can configure the router to match the entire IP address or just a part of the IP address. When defining the ACL statements you can define a wildcard mask along with the IP address. The wildcard mask tells the router which part of the IP address in the configuration statement must be compared with the packet header. The wildcard masks look similar to subnet masks, in that they represent a 32-bit number. However, the wildcard mask’s 0 bits tell the router that those corresponding bits in the address must be compared when performing the matching logic. The binary 1s in the wildcard mask tell the router that those bits do not need to be compared. Thus, wildcard mask 0.0.0.0, which in binary form is 00000000.00000000.00000000.00000000, indicates that the entire IP address must be matched, while wildcard mask 0.0.0.255, which in binary form is 00000000.00000000.00000000.11111111, indicates that the first 24 bits of the IP address must be matched, and wildcard mask 0.0.31.255, which in binary form is 00000000.00000000.00011111.11111111, indicates that the first 24 bits of the IP address must be matched.
Standard IP Access List Configuration
A standard access list is used to match a packet and then take the directed action. Each standard ACL can match all, or only part, of the packet’s source IP address. The only two actions taken when an access-list statement is matched are to either deny or permit the packet.
The configuration commands required are:
. ip access-group {number | action [in | out]}, in which action can be either permit of deny and is used to enable access lists; and
. access-class number | action [in | out], which can be used to enable either standard or extended access lists.
The standard access list configuration can be verified using the following show commands:
. show ip interface[type number], which includes a reference to the access lists enabled on the interface;
. show access-lists [access-list-number | access-list-name], which shows details of configured access lists for all protocols; and
. show ip access-list [access-list-number | access-list-name], which shows the access lists.
Extended IP Access Control Lists
Extended IP access lists are similar to standard IP ACLs in that you enable extended access lists on interfaces for packets either entering or exiting the interface. IOS then searches the list sequentially. The first statement matched stops the search through the list and defines the action to be taken. The key difference between the extended ACLs and standard ACLs is the variety of fields in the packet that can be compared for matching by extended access lists. A single extended ACL statement can examine multiple parts of the packet headers, requiring that all the parameters be matched correctly in order to match that one ACL statement. That matching logic is what makes extended access lists both much more useful and much more complex than standard IP ACLs. You can configure extended ACL to match the IP protocol type, which identifies what header follows the IP header. You can specify all IP packets, or those with TCP headers, UDP headers, ICMP, etc, by checking the Protocol field. You can also check the source and destination IP addresses, as well as the TCP source and destination port numbers.
An extended access list is more complex than standard access lists. Therefore the configuration commands are more complex. The configuration command for extended access lists is:
. access-list access-list-number action protocol source source-wildcard destination destination-wildcard [log | log-input], which can be used to enable access lists;
Named IP Access Lists
Named ACLs can be used to match the same packets, with the same parameters, you can match with standard and extended IP ACLs. Named IP ACLs do have some differences, however. The most obvious difference is that IOS identifies named ACLs using names you assign them as opposed to numbers. Named ACLs also have another key feature that numbered ACLs do not: You can delete individual lines in a named IP access list.
In addition, two important configuration differences exist between numbered and named access lists. One key difference is that named access lists use a global command that places the user in a named IP access list submode, under which the matching and permit or deny logic is configured. The other key difference is that when a named matching statement is deleted, only that one statement is deleted. With numbered lists, the deletion of any statement in the list deletes all the statements in the list.
Controlling Telnet Access with ACLs
Access into and out of the virtual terminal line (vty) ports of the Cisco IOS software can also be controlled by IP access lists. IOS uses vtys to represent a user who has Telnetted to a router, as well as for Telnet sessions a user of a router has created to other devices. You can use ACLs to limit the IP hosts that can Telnet into the router, and you can also limit the hosts to which a user of the router can Telnet.
Filtering logic could be configured on any router and on any of its interfaces. Cisco IOS software applies the filtering logic of an ACL either as a packet enters an interface or as it exits the interface. In other words, IOS associates an ACL with an interface, and specifically for traffic either entering or exiting the interface. After you have chosen the router on which you want to place the access list, you must choose the interface on which to apply the access logic, as well as whether to apply the logic for inbound or outbound packets.
The key features of Cisco ACLs are:
. Packets can be filtered as they enter an interface, before the routing decision.
. Packets can be filtered before they exit an interface, after the routing decision.
. Deny is the term used in Cisco IOS software to imply that the packet will be filtered.
. Permit is the term used in Cisco IOS software to imply that the packet will not be filtered.
. The filtering logic is configured in the access list.
. If a packet does not match any of your access list statements, it is blocked.
Access lists have two major steps in their logic: matching, which determines whether it matches the access-list statement; and action, which can be either deny or permit. Deny means to discard the packet, and permit implies that the packet should be allowed. However, the logic that IOS uses with a multiple-entry ACL can be much more complex. Generally, the logic can be summarized as follows:
Step 1: The matching parameters of the access-list statement are compared to the packet.
Step 2: If a match is made, the action defined in this access-list statement (permit or deny) is performed.
Step 3: If a match is not made in Step 2, repeat Steps 1 and 2 using each successive statement in the ACL until a match is made.
Step 4: If no match is made with an entry in the access list, the deny action is performed.
Wildcard Masks
IOS IP ACLs match packets by looking at the IP, TCP, and UDP headers in the packet. Standard IP access lists can also examine only the source IP address. You can configure the router to match the entire IP address or just a part of the IP address. When defining the ACL statements you can define a wildcard mask along with the IP address. The wildcard mask tells the router which part of the IP address in the configuration statement must be compared with the packet header. The wildcard masks look similar to subnet masks, in that they represent a 32-bit number. However, the wildcard mask’s 0 bits tell the router that those corresponding bits in the address must be compared when performing the matching logic. The binary 1s in the wildcard mask tell the router that those bits do not need to be compared. Thus, wildcard mask 0.0.0.0, which in binary form is 00000000.00000000.00000000.00000000, indicates that the entire IP address must be matched, while wildcard mask 0.0.0.255, which in binary form is 00000000.00000000.00000000.11111111, indicates that the first 24 bits of the IP address must be matched, and wildcard mask 0.0.31.255, which in binary form is 00000000.00000000.00011111.11111111, indicates that the first 24 bits of the IP address must be matched.
Standard IP Access List Configuration
A standard access list is used to match a packet and then take the directed action. Each standard ACL can match all, or only part, of the packet’s source IP address. The only two actions taken when an access-list statement is matched are to either deny or permit the packet.
The configuration commands required are:
. ip access-group {number | action [in | out]}, in which action can be either permit of deny and is used to enable access lists; and
. access-class number | action [in | out], which can be used to enable either standard or extended access lists.
The standard access list configuration can be verified using the following show commands:
. show ip interface[type number], which includes a reference to the access lists enabled on the interface;
. show access-lists [access-list-number | access-list-name], which shows details of configured access lists for all protocols; and
. show ip access-list [access-list-number | access-list-name], which shows the access lists.
Extended IP Access Control Lists
Extended IP access lists are similar to standard IP ACLs in that you enable extended access lists on interfaces for packets either entering or exiting the interface. IOS then searches the list sequentially. The first statement matched stops the search through the list and defines the action to be taken. The key difference between the extended ACLs and standard ACLs is the variety of fields in the packet that can be compared for matching by extended access lists. A single extended ACL statement can examine multiple parts of the packet headers, requiring that all the parameters be matched correctly in order to match that one ACL statement. That matching logic is what makes extended access lists both much more useful and much more complex than standard IP ACLs. You can configure extended ACL to match the IP protocol type, which identifies what header follows the IP header. You can specify all IP packets, or those with TCP headers, UDP headers, ICMP, etc, by checking the Protocol field. You can also check the source and destination IP addresses, as well as the TCP source and destination port numbers.
An extended access list is more complex than standard access lists. Therefore the configuration commands are more complex. The configuration command for extended access lists is:
. access-list access-list-number action protocol source source-wildcard destination destination-wildcard [log | log-input], which can be used to enable access lists;
Named IP Access Lists
Named ACLs can be used to match the same packets, with the same parameters, you can match with standard and extended IP ACLs. Named IP ACLs do have some differences, however. The most obvious difference is that IOS identifies named ACLs using names you assign them as opposed to numbers. Named ACLs also have another key feature that numbered ACLs do not: You can delete individual lines in a named IP access list.
In addition, two important configuration differences exist between numbered and named access lists. One key difference is that named access lists use a global command that places the user in a named IP access list submode, under which the matching and permit or deny logic is configured. The other key difference is that when a named matching statement is deleted, only that one statement is deleted. With numbered lists, the deletion of any statement in the list deletes all the statements in the list.
Controlling Telnet Access with ACLs
Access into and out of the virtual terminal line (vty) ports of the Cisco IOS software can also be controlled by IP access lists. IOS uses vtys to represent a user who has Telnetted to a router, as well as for Telnet sessions a user of a router has created to other devices. You can use ACLs to limit the IP hosts that can Telnet into the router, and you can also limit the hosts to which a user of the router can Telnet.
CISCO IOS DHCP
To configure Cisco IOS DHCP, follow these steps, which include sample commands:
1. Configure an IP address on the router's Ethernet port, and bring up the interface. (On an existing router, you would have already done this.)
Router(config)# interface ethernet0/0
Router(config-if)#ip address 1.1.1.1 255.0.0.0
Router(config-if)# no shutdown
2. Create a DHCP IP address pool for the IP addresses you want to use.
Router(config)# ip dhcp pool mypool
3. Specify the network and subnet for the addresses you want to use from the pool.
Router(dhcp-config)# network 1.1.1.0 /8
4. Specify the DNS domain name for the clients.
Router(dhcp-config)#domain-name mydomain.com
5. Specify the primary and secondary DNS servers.
Router(dhcp-config)#dns-server 1.1.1.10 1.1.1.11
6. Specify the default router (i.e., default gateway).
Router(dhcp-config)#default-router 1.1.1.1
7. Specify the lease duration for the addresses you're using from the pool.
Router(dhcp-config)#lease 7
8. Exit Pool Configuration Mode.
Router(dhcp-config)#exit
1. Configure an IP address on the router's Ethernet port, and bring up the interface. (On an existing router, you would have already done this.)
Router(config)# interface ethernet0/0
Router(config-if)#ip address 1.1.1.1 255.0.0.0
Router(config-if)# no shutdown
2. Create a DHCP IP address pool for the IP addresses you want to use.
Router(config)# ip dhcp pool mypool
3. Specify the network and subnet for the addresses you want to use from the pool.
Router(dhcp-config)# network 1.1.1.0 /8
4. Specify the DNS domain name for the clients.
Router(dhcp-config)#domain-name mydomain.com
5. Specify the primary and secondary DNS servers.
Router(dhcp-config)#dns-server 1.1.1.10 1.1.1.11
6. Specify the default router (i.e., default gateway).
Router(dhcp-config)#default-router 1.1.1.1
7. Specify the lease duration for the addresses you're using from the pool.
Router(dhcp-config)#lease 7
8. Exit Pool Configuration Mode.
Router(dhcp-config)#exit
Thursday, June 18, 2009
IP SEC
IPSec is an suit of protocols designed to provide interopable and high secure data transfer service. to understand IPSec we need to go to the basics and see some defenitions and protocols used by IPSec and start from there to build our understanding on IPSec, after we know what is IPSec and what he need to provide us we can go over to the practical usage and some configuration samples.
so as I have said we have some basics to cover and we will start with:
Authentication - is how the units verify they are who they say they are
Data Integrity - making sure that the data that was sent it what was recived in the other side with no change
Confidentiality - it the Encription of the data
Anti-Replay - preventing play back attack, if this mechanizem was not enable then a potential attacker could capture a stream of data and replay it to the box this stream was sent and potentialy could log into the network even if the data is hashed it dosent metter as the other side need to know to unpack that data.
this 4 definitios are the very basic to understand, and each one is playing a very important role in the vpn.
AH Authentication Header - as it is mentioned in his name it is a header authentication method and can provide integrity authentication and anti-relplay, it is the older form of creating IPSec VPN, and today less used.
ESP encapsulation security protocol - this is the new form of creatign IPSec VPN and it add the very important element of Confidentiality or encription of the data as I mentioned.
the methods we have to encrypt the data are very wide spread but here are the most common ones
DES data encription standart 64bit key
3DES it is 192bit what is even funy The procedure for encryption is exactly the same as regular DES, but it is repeated three times.
AES Advanced Encryption Standard has a minimum key size of 128bit and maximum of 256bit, a AES 128 is considered more secure then
3DES.
RSA (Ron Rivest, Adi Shamir, and Leonard Adleman) is used for Asymetric Public Private Keys
Authentication there are 2 main methods to authenticate
pre-shared key is a staticly defigned by the Admin on the units the less secure way but the more common method
Certificate Authority this is the high security methode and the less common due to the complex of configuration and usually also you need to buy Certificate from one of the vendors like verisign, commodo...
Integrity is using hashing for making sure that that the data is not changed:
MD5 Message-Digest algorithm 5 the most commonly hash used today the hash size is 128bit.
SHA-1 Secure Hash Algorithm 1 the hash size is 160bit
DH Diffie-Hellman "A public-key cryptography protocol which allows two parties to establish a shared secret over an insecure communications channel. Diffie-Hellman is used within IKE to establish session keys and is a component of Oakley." (this line was taken from Cisco Site: http://www.cisco.com/en/US/docs/ios/12_1t/12_1t3/feature/guide/dtgroup5.html#wp1015327 )
Let Me try to expalin the proccess;
each unit have a private key (used for decryption) a key that is never passed, and a Diffie-Helman Key (Public Key used for encryption) when a unit want to do a key exchange they each send there Public Key to the other side so lets drill down to Unit_A, Unit_A get the Public Key of Unit_B then using the RSA create a shared key that shared key can only be opened on Unit_B with Unit_B Private Key so even if you intercept the shared key you cant reverse engineer it to see as only the private key of Unit_B will be able to understand it.
ok untill here I have summerized for you all the key concepts and provided an example of the proccess used in Asymetric Process of the IPSec next I will take the concept and show you in practice what need to be done to form an IPSec connection. General Guide lines to configure an IPSec connection:
1) Create IKE Policy
2) Create IPsec Transform Set
3) Defign ACL for the encription
4) Configure a Crypto Map
5) Assign the Crypto Map to an Interface
Note: when you want to create an IPsec between 2 units you must make sure there configuration match so this is a tip copy the configuration you did to a notepad and on the other side unit only flip the ACL IP address to match the other side and paset it, if you did correct on the first side you will have a working connection, if you did bad then you will need to troubleshoot only one side and again copy paste to the other side, save time and pain!
so as I have said we have some basics to cover and we will start with:
Authentication - is how the units verify they are who they say they are
Data Integrity - making sure that the data that was sent it what was recived in the other side with no change
Confidentiality - it the Encription of the data
Anti-Replay - preventing play back attack, if this mechanizem was not enable then a potential attacker could capture a stream of data and replay it to the box this stream was sent and potentialy could log into the network even if the data is hashed it dosent metter as the other side need to know to unpack that data.
this 4 definitios are the very basic to understand, and each one is playing a very important role in the vpn.
AH Authentication Header - as it is mentioned in his name it is a header authentication method and can provide integrity authentication and anti-relplay, it is the older form of creating IPSec VPN, and today less used.
ESP encapsulation security protocol - this is the new form of creatign IPSec VPN and it add the very important element of Confidentiality or encription of the data as I mentioned.
the methods we have to encrypt the data are very wide spread but here are the most common ones
DES data encription standart 64bit key
3DES it is 192bit what is even funy The procedure for encryption is exactly the same as regular DES, but it is repeated three times.
AES Advanced Encryption Standard has a minimum key size of 128bit and maximum of 256bit, a AES 128 is considered more secure then
3DES.
RSA (Ron Rivest, Adi Shamir, and Leonard Adleman) is used for Asymetric Public Private Keys
Authentication there are 2 main methods to authenticate
pre-shared key is a staticly defigned by the Admin on the units the less secure way but the more common method
Certificate Authority this is the high security methode and the less common due to the complex of configuration and usually also you need to buy Certificate from one of the vendors like verisign, commodo...
Integrity is using hashing for making sure that that the data is not changed:
MD5 Message-Digest algorithm 5 the most commonly hash used today the hash size is 128bit.
SHA-1 Secure Hash Algorithm 1 the hash size is 160bit
DH Diffie-Hellman "A public-key cryptography protocol which allows two parties to establish a shared secret over an insecure communications channel. Diffie-Hellman is used within IKE to establish session keys and is a component of Oakley." (this line was taken from Cisco Site: http://www.cisco.com/en/US/docs/ios/12_1t/12_1t3/feature/guide/dtgroup5.html#wp1015327 )
Let Me try to expalin the proccess;
each unit have a private key (used for decryption) a key that is never passed, and a Diffie-Helman Key (Public Key used for encryption) when a unit want to do a key exchange they each send there Public Key to the other side so lets drill down to Unit_A, Unit_A get the Public Key of Unit_B then using the RSA create a shared key that shared key can only be opened on Unit_B with Unit_B Private Key so even if you intercept the shared key you cant reverse engineer it to see as only the private key of Unit_B will be able to understand it.
ok untill here I have summerized for you all the key concepts and provided an example of the proccess used in Asymetric Process of the IPSec next I will take the concept and show you in practice what need to be done to form an IPSec connection. General Guide lines to configure an IPSec connection:
1) Create IKE Policy
2) Create IPsec Transform Set
3) Defign ACL for the encription
4) Configure a Crypto Map
5) Assign the Crypto Map to an Interface
Note: when you want to create an IPsec between 2 units you must make sure there configuration match so this is a tip copy the configuration you did to a notepad and on the other side unit only flip the ACL IP address to match the other side and paset it, if you did correct on the first side you will have a working connection, if you did bad then you will need to troubleshoot only one side and again copy paste to the other side, save time and pain!
MPLS Deployment reasons
1) Faster convergence, in the old days that was a valid reason due to the relatively complex forwarding task that required more resources then Label forwarding. Today non relevant
2) RFC 1483 the newer 2684 AAL5 ATM Adaptation Layer 5 the implementation of IP over ATM
3) BGP Core Free on the SP network, as with MPLS lookup is done based on Labels and not destination address there is no need to have the
BGP table in the Core for external prefixes lookup's. this is a massive change from the requirement that
Every router in the core must have BGP enabled (cpu and memory intensive load) to only the edge router have BGP enabled mean higher performance
and capability.
Note: edge routers still need to have the BGP routing tables, edge routers are translating between ip routing decision to label based decisions.
4) Deployment scalability, when we face with large scale client to deploy (client that connect with 50 - 300 sites and more) we need to
have under the consideration the deployment scalability and management, with that in mind 2 models of deployment are optional:
a) VPN Overlay - creating a point to point connection over the SP network, can be achieved in layer 1, 2 or 3.
Layer 1 TDM E1 T1...
Layer 2 ATM FR...
Layer 3 GRE, IPIP...
b) Peer to Peer - creating a connection between sites trough the ISP and with him, what I mean is that the SP need to
join the client network and to achieve client privacy the SP need to manage acl's and routing updates, not very scalable and
a lot of overhead. notice that in addition to the disadvantage for the SP on the additional management overhead and complexity
there is the client control (doesn't have any) of his layer 3 network trough the SP.
With MPLS the VPN allow the Peer to Peer bad model to have advantage over the Overlay model where in the MPLS we use VRF
Virtual Routing Forwarding separators between each network and the configuration is done only on each new site.
meaning that if I am an SP and I have 3 client (Cisco, Microsoft, Verizon) each vrf will have a unique color vrf Cisco, vrf Microsoft and
vrf Verizon, and to join a new branch is only to color this branch traffic accordingly, so the main work is done in the initial design and
implementation and any new addition is actually very simple to add.
5) TE - traffic engineering is a small phrase for a very big spectrum of options, normally traffic routing is decided at each point
separately and usually the best route is chosen according to the shortest path to destination, using TE we can make the routing
decision based on multiple criteria options. allowing the traffic to fully utilize network capability.
FRR - Fast ReRouting is a very good feature that allow you to detect and reroute based on router availability in less than 50ms
Very important in high sensitive traffic like VoIP.
Monday, June 15, 2009
CCNP INTERVIEW QUESTIONS:
Quick Notes - LAN
What is carrier sense multiple access collision detect (CSMA/CD)?
CSMA/CD describes the Ethernet access method. In CSMA/CD, many stations can transmit on the same cable, and no station has priority over any other. Before a station transmits, it listens on the wire to make sure no other station is transmitting. If no other station is transmitting, the station transmits across the wire. CSMA/CD is all about devices taking turns using the wire.
What are MAC addresses?
For computers to identify each other on the data link layer, they need a MAC address (hardware address). All devices on a LAN must have a unique MAC address. A MAC address is a 48-bit (six octet) address burned into a network interface card. The first three octets (24 bits) of the MAC address indicate the vendor that manufactured the card. This is called the Organization Unique Identifier (OUI). The last three octets of the MAC address are the unique host address. An example of a MAC address is 00-80-C6-E7-9C-EF.
What are the three types of LAN traffic?
The three types of LAN traffic are:
Unicasts
Broadcasts
Multicasts
What are unicast frames?
Unicast frames are the most common type of LAN traffic. A unicast frame is a frame intended for only one host. In unicast frames, the only station that processes the frame is the station that has its own MAC address in the destination portion of the packet.
What are broadcast frames?
Broadcast frames are frames intended for everyone. Stations view broadcast frames as public service announcements. All stations receive and process broadcast frames. In large networks, broadcasts can bring the network to a crawl, because every computer must process them.
What is the destination address of broadcast frames?
The destination address of broadcast frames (Layer 2 broadcast addresses) is FF-FF-FF-FF-FF-FF, or all 1s in binary.
What are multicast frames?
Multicast frames address a group of devices that have a common interest. These frames allow the source to send only one copy of the frame on the network even though it is intended for several stations. Only stations that have a card that is configured to receive multicast frames process them. All other stations discard multicast frames.
What devices can you use to segment a LAN at Layer 1, Layer 2, and Layer 3?
Three devices you can use to segment a LAN are:
Hubs/repeaters (Layer 1)
Bridges/switches (Layer 2) - physical addresses
Routers (Layer 3) - logical addresses
What happens when you segment the network with hubs/repeaters?
Because hubs and repeaters operate at the physical layer of the OSI model, segmenting a network with these devices appears as an extension to the physical cable. Hubs and repeaters are transparent to devices. They are unintelligent devices. All devices that connect to a hub/repeater share the same bandwidth. Hubs/repeaters create a single broadcast and collision domain.
What is the advantage of segmenting a network with bridges/switches?
Bridges/switches operate at Layer 2 of the OSI model and filter by MAC address. Each port on a bridge/switch provides full-dedicated bandwidth and creates a single collision domain. Because bridges/switches operate at Layer 2 of the OSI model, they cannot filter broadcasts, and they create a single broadcast domain. For the CCNA test, remember that switches create more collision domains and fewer collisions.
What is the difference between bridges and switches?
Bridges and switches function the same way; the only difference is in how they are implemented. Bridges are implemented by software and usually have a couple of network ports. Switches are implemented in hardware by ASIC chips and have many ports.
What are the advantages and disadvantages of segmenting the LAN with routers?
An advantage of segmenting the LAN with routers is that each interface on a router creates a single broadcast and collision domain. Routers operate at Layer 3 of the OSI model and do not propagate broadcasts. Some disadvantages are that routers are not transparent and are implemented in software, thus introducing latency in the network.
What is the Maximum Transmission Unit (MTU) for an Ethernet frame?
1500 bytes is the MTU for an Ethernet frame. You will notice that some publications state that the MTU for Ethernet is 1518 bytes. This is correct also. But what is the true answer? The MTU for Ethernet, including the header, source and destination address, data, and CRC is 1518 bytes. The MTU for the data portion of the frame is 1500 bytes.
What three major functions do Layer 2 switches provide?
The three major functions that Layer 2 switches provide are
Address learning
Packet forwarding/filtering
Loop avoidance by spanning tree
What are some advantages of switches?
Some advantages of switches are as follows:
They increase available network bandwidth.
They reduce the number of users per segment.
They provide dedicated bandwidth to each segment.
Transparent bridging (switching) provides five bridging functions to determine what to do when it receives a frame.
What are these five processes?
The five processes are:
Learning
Flooding
Filtering
Forwarding
Aging
In transparent bridging, what is the learning process?
The first process a bridge goes through when it is powered on is the learning process. The MAC address table on the bridge contains no entries, and the bridge goes through the learning process to record all workstations on every interface. In the learning process, the bridge records the source MAC address and source port number in the MAC address table every time it sees a frame.
In transparent bridging, what is the flooding process?
When a bridge is first turned on, it has no MAC address in its table. When a switch receives a unicast frame, it knows the source address and port from which the unicast frame came, but no entry exists in its table for the destination address. This is called an unknown unicast frame. When a switch receives an unknown unicast frame, it sends the frame out all forwarding interfaces on the bridge except the interface that received the frame. This process is the flooding process.
In transparent bridging, what is the filtering process?
The filtering process occurs when the source and destination addresses reside on the same interface on the bridge. Because the bridge does not need to forward a frame in which the destination and source addresses reside on the same interface, it filters the frame and discards it.
In transparent bridging, what is the forwarding process?
The forwarding process occurs when a switch receives a unicast frame and has an entry of the destination address in its MAC table. The switch then forwards the frame to the interface where that destination address resides.
In transparent bridging, what occurs during the aging process?
Every time a bridge learns a source address, it time-stamps the entry. When the bridge sees a frame from this source, it updates the time stamp. If the bridge does not hear from the source for a specific amount of time (called the aging timer), the bridge deletes the entry from its MAC address table. This process is the aging process.
What is the default aging time in transparent bridges?
The default aging timer is 5 minutes.
What is the Spanning-Tree Protocol (STP)?
STP is a loop-prevention bridge-to-bridge protocol. Its main purpose is to dynamically maintain a loop-free network. It does this by sending out Bridge Protocol Data Units (BPDUs), discovering any loops in the topology, and blocking one or more redundant links.
How does STP maintain a loop-free network?
STP maintains a loop-free network by
Electing a root bridge
Electing a root port on each nonroot bridge
Electing designated ports
Putting in the blocking state any port that is not a root port or designated port
In spanning tree, what is a Bridge ID (BID)?
A BID is an 8-byte field that is composed of the bridge's 6-byte MAC address and a 2-byte bridge priority.
What is the default bridge priority in a Bridge ID for all Cisco switches?
32,768
In spanning tree, what is path cost?
Path cost is a calculation to determine the link's bandwidth. It is a value assigned to each port that is based on the port's speed.
What is the spanning tree path cost for each of the following?
10 Mbps
100 Mbps
1 Gbps
The path costs are as follows:
10 Mbps - 100
100 Mbps - 19
1 Gbps - 4
When calculating a loop-free environment, what four-step decision sequence does spanning tree use to determine what will be the root bridge and which ports will forward or block?
The four-step decision sequence that spanning tree uses to determine the root bridge and which port will forward is as follows:
Step 1. The lowest root BID
Step 2. The lowest path cost to the root bridge
Step 3. The lowest sender BID
Step 4. The lowest port ID
How do bridges pass spanning tree information between themselves?
Bridges pass STP information using special frame called Bridge Protocol Data Units (BPDUs).
How often do bridges send BPDUs out active ports?
The default time that bridges send BPDUs out active ports is 2 seconds.
Note: All ports on a switch listen for BPDUs in case there is a topology change.
In STP, how is a root bridge elected?
In STP, the bridge with the lowest BID is elected the root bridge. All ports on the root bridge are placed in the forwarding state and are called designated ports.
Note: The BID is a 6-byte field that is composed of a default priority (32,768) and a MAC address. Because all Cisco switches use the default priority, the switch with the lowest MAC address is elected the root bridge. As a rule of thumb, lower will always win in spanning tree.
After bridges elect the root bridge, what do they do next?
After electing the root bridge, switches elect root ports. A root port is the port on nonroot bridges that is closest to the root bridge. Every nonroot bridge must select one root port.
How do nonroot bridges decide which port they will elect as a root port?
Nonroot bridges use root path cost to determine which port will be the root port. Root path cost is the cumulative cost of all links to the root bridge. The port with the lowest root path cost is elected the bridge's root port and is placed in the forwarding state.
What is the difference between path cost and root path cost?
Path cost is the value assigned to each port. It is added to BPDUs received on that port to calculate the root path cost. Root path cost is defined as the cumulative cost to the root bridge. In a BPDU, this is the value transmitted in the cost field. In a bridge, this value is calculated by adding the receiving port's path cost to the value contained in the BPDU.
If a nonroot bridge has two redundant ports with the same root path cost, how does the bridge choose which port will be the root port?
If a nonroot bridge has redundant ports with the same root path cost, the deciding factor is the port with the lowest port ID (port number).
After the root bridge and root ports are selected, the last step in spanning tree is to elect designated ports. How do bridges elect designated ports?
In spanning tree, each segment in a bridged network has one designated port. This port is a single port that both sends and receives traffic to and from that segment and the root bridge. All other ports are placed in a blocking state. This ensures that only one port on any segment can send and receive traffic to and from the root bridge, ensuring a loop-free topology. The bridge containing the designated port for a segment is called the designated bridge for that segment. Designated ports are chosen based on cumulative root path cost to the root bridge.
Note: Every active port on the root bridge becomes a designated port.
If a bridge is faced with a tie in electing designated ports, how does it decide which port will be the designated port?
In the event of a tie, STP uses the four-step decision process discussed in Question 30. It first looks for the BPDU with the lowest BID; this is always the root bridge. If the switch is not the root bridge, it moves to the next step: the BPDU with the lowest path cost to the root bridge. If both paths are equal, STP looks for the BPDU with the lowest sender BID. If these are equal, STP uses the link with the lowest port ID as the final tiebreaker.
What are the four spanning tree port states?
The four spanning tree port states are
Blocking
Listening
Learning
Forwarding
Remember that root and designated ports forward traffic and that nondesignated ports block traffic but still listen for BPDUs.
Important note: There is another port state - Disabled - (No frames forwarded, no BPDUs heard). If it shows up in the answer options - select it along with the others.
What is the STP blocking state?
When a switch starts, all ports are in the blocking state. This is to prevent any loops in the network. If there is a better path to the root bridge, the port remains in the blocked state. Ports in the blocked state cannot send or receive traffic, but they can receive BPDUs.
What is the STP listening state?
Ports transition from a blocked state to a listening state. In this state, no user data is passed. The port only listens for BPDUs. After listening for 15 seconds (if the bridge does not find a better path), the port moves to the next state, the learning state.
What is the STP learning state?
In the STP learning state, no user data is being passed. The port quietly builds its bridging table. The default time in the learning state is 15 seconds.
What is the STP forwarding state?
After the default time in the learning state is up, the port moves to the forwarding state. In the forwarding state, the port sends and receives data.
What is STP forward delay?
The forward delay is the time it takes for a port to move from the listening state to the learning state or from the learning state to the forwarding state. The default time is 30 seconds.
What is the hello time in STP timers?
The hello time is the time interval between the sending of BPDUs. The default time is 2 seconds.
What is the Max Age timer?
The Max Age timer is how long a bridge stores a BPDU before discarding it. The default time is 20 seconds (ten missed hello intervals).
What is the default time a port takes to transition from the blocking state to the forwarding state?
The default time a port takes to transition from the blocking state to the forwarding state is 50 seconds: 20 seconds for Max Age, 15 seconds for listening, and 15 seconds for learning.
What does STP do when it detects a topology change in the network due to a bridge or link failure?
If spanning tree detects a change in the network due to a bridge or link failure, at least one bridge interface changes from the blocking state to the forwarding state, or vice versa.
WAN
The three WAN connection types available are leased lines, circuit-switched, and packet-switched. Define the differences between each connection type.
Leased lines are dedicated point-to-point lines that provide a single preestablished WAN communication path from the customer's network to the remote network. Leased lines are usually employed over synchronous connections. They are generally expensive and are always up.
Circuit-switched connections are dedicated for only the duration of the call. The telephone system and ISDN are examples of circuit-switched networks. Packet-switched connections use virtual circuits (VCs) to provide end-to-end connectivity.
Packet-switched connections are similar to leased lines, except that the line is shared by other customers. A packet knows how to reach its destination by programming of switches. Frame Relay is an example of a packet-switched connection.
Define customer premises equipment (CPE), and give an example.
CPE is equipment that is located on the customer's (or subscriber's) premises. It is equipment owned by the customer or equipment leased by the service provider to the customer. An example is your router.
What is the demarcation point (demarc)?
The demarc is the point where the CPE ends and the local loop begins. It is the last responsibility of the service provider and is usually an RJ-45 jack located close to the CPE. Think of the demarc as the boundary between the customer's wiring and the service provider's wiring.
What is the local loop?
The local loop is the physical cable that extends from the demarc to the central office.
Describe the central office (CO).
The CO is the telco switching facility that connects the customer to the provider's switching network. The CO is sometimes referred to as a point of presence. It is the point where the local loop gains access to the service provider's access lines.
What is the toll network?
All the telco switches, COs, and trunk lines inside the WAN provider's network are the toll network.
What are synchronous links?
Synchronous links have identical frequencies and contain individual characters encapsulated in control bits, called start/stop bits, that designate the beginning and end of each character. Synchronous links try to use the same speed as the other end of a serial link.
What are Asynchronous links?
Asynchronous links send digital signals without timing. Asynchronous links agree on the same speed, but there is no check or adjustment of the rates if they are slightly different. Only 1 byte per transfer is sent.
List some typical Layer 2 encapsulation methods for WAN links.
. High-Level Data Link Control (HDLC)
• Point-to-Point Protocol (PPP)
• Serial Line Internet Protocol (SLIP)
• X.25/Link Access Procedure, Balanced (LAPB)
• Frame Relay• Asynchronous Transfer Mode (ATM)
Describe HDLC.
HDLC was derived from Synchronous Data Link Control (SDLC). It is the default encapsulation type on point-to-point dedicated links and circuit-switched connections between Cisco routers. It is an ISO-standard bit-oriented data-link protocol that encapsulates data on synchronous links. HDLC is a connection-oriented protocol that has very little overhead. HDLC lacks a protocol field and therefore cannot encapsulate multiple network layer protocols across the same link. Because of this, each vendor has its own method of identifying the network-layer protocol. Cisco offers a propriety version of HDLC that uses a type field that acts as a protocol field, making it possible for multiple network-layer protocols to share the same link.
By default, Cisco uses HDLC as its default encapsulation method across synchronous lines (point-to-point links). If a serial line uses a different encapsulation protocol, how do you change it back to HDLC?
To change a serial line back to HDLC, use the following interface command on the serial interface you want to change: Router(config-if)#encapsulation hdlc
What is the Point-to-Point Protocol (PPP)?
PPP is an industry-standard protocol that provides router-to-router or router-to-host connections over synchronous and asynchronous links. It can be used to connect to other vendors' equipment. It works with several network-layer protocols, such as IP and IPX. PPP provides authentication through PAP or CHAP.
Describe X.25/LAPB.
X.25/LAPB is an ITU-T standard that has a tremendous amount of overhead because of its strict timeout and windowing techniques. LAPB is the connection-oriented protocol used with X.25. It uses the ABM (Asynchronous Balance Mode) transfer mode. X.25/LAPB was used in the 1980s when WAN links were not as error-free as they are today. X.25 is a predecessor of Frame Relay. X.25 supports both switched and permanent virtual circuits.
What is Frame Relay?
An industry standard, Frame Relay is a switched data link layer protocol that uses virtual circuits to identify the traffic that belongs to certain routers. It provides dynamic bandwidth allocation and congestion control.
Quick Notes - INTERNETWORKING
What are the three layers of the Cisco Hierarchical Model?
The three layers of the Cisco Hierarchical Model are: 1
the access layer
The distribution layer
The core layer
In the Cisco Hierarchical Model, what is the function of the access layer?
Sometimes referred to as the desktop layer, the access layer is the point at which users connect to the network through low-end switches. Some functions of the access layer include:
Connectivity into the distribution layer
Shared Bandwidth
MAC Address filtering (switching)
Segmentation
What is the function of the distribution layer in the Cisco Hierarchical Model?
The distribution layer is also known as the workgroup layer. It is the demarcation point between the access and core layers of the network. Its primary function is to provide routing, filtering, and WAN access. The distribution layer determines how packets access the core, so it is the layer at which to implement policy-based connectivity. Some functions include the following:
Collection point for access layer devices
Broadcast and multicast domain segmentation
Security and filtering services such as firewalls and access lists
Providing translation between different media types
Inter-VLAN routing
What is the role of the core layer in the Cisco Hierarchical Model?
The core layer is the backbone of the network. Its main function is to switch traffic as fast as possible. Therefore, it should not perform any filtering to slow down traffic.
The ISO's OSI Reference Model contains seven layers. What are they? Include the layer number and name of each layer in your answer.
The seven layers of the OSI model are as follows:
Layer 7 - Application layer
Layer 6 - Presentation layer
Layer 5 - Session layer
Layer 4 - Transport layer
Layer 3 - Network layer
Layer 2 - Data link layer
Layer 1 - Physical layer
What are some reasons that the industry uses a layered model?
Here are some reasons why the industry uses a layered model:
It encourages industry standardization by defining what functions occur at each level.
It allows vendors to modify or improve components at only one layer versus rewriting the whole protocol stack.
It helps interoperability by defining standards for the operations at each level.
It helps with troubleshooting.
What does the application layer (Layer 7) of the OSI model do, and what are some examples of this layer?
The application layer is the layer that is closest to the user. This means that this layer interacts directly with the software application. The application layer's main function is to identify and establish communication partners, determine resource availability, and synchronize communication. Some examples include the following:
TCP/IP applications such as Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), WWW, and HTTP.
OSI applications such as Virtual Terminal Protocol, File
Transfer, Access, and Management (FTAM), and Common Management Information Protocol (CMIP).
In the OSI model, what are the responsibilities of the presentation layer (Layer 6)? Give some examples of this layer.
Also known as the translator, the presentation layer provides coding and conversion functions to application layer data. This guarantees that the application layer on another system can read data transferred from the application layer of a different system. Some examples of the presentation layer are:
Compression, decompression, and encryption
JPEG, TIFF, GIFF, PICT, QuickTime, MPEG, and ASCII
What are the functions of the session layer (Layer 5)? Give some examples.
The session layer is responsible for creating, managing, and ending communication sessions between presentation layer entities. These sessions consist of service requests and responses that develop between applications located on different network devices. Some examples include SQL, RPC, NFS, X Window System, ZIP, NetBIOS names, and AppleTalk ASP.
What is the transport layer (Layer 4) responsible for? Give some examples of transport layer implementations.
The transport layer segments and reassembles data from upper-layer applications into data streams. It provides reliable data transmission to upper layers. End-to-end communications, flow control, multiplexing, error detection and correction, and virtual circuit management are typical transport layer functions. Some examples include TCP, UDP*, and SPX.
Note: watch out for end-to-end on communications on the exam! Transport layer.
* Error correction does not apply to UDP - connection-less - unreliable.....
What is flow control, and what are the three methods of implementing it?
Flow control is the method of controlling the rate at which a computer sends data, thus preventing network congestion. The three methods of implementing flow control are
Buffering
Congestion avoidance
Windowing
Almost certain to be on the exam.
Describe the function of the network layer (Layer 3), and give some examples of network layer implementations.
The network layer provides internetwork routing and logical network addresses. It defines how to transport traffic between devices that are not locally attached. The network layer also supports connection-oriented and connectionless service from higher-layer protocols. Routers operate at the network layer. IP, IPX, AppleTalk, and DDP are examples of network layer implementations.
Are network layer addresses physical or logical?
Network layer addresses are logical addresses specific to the network layer protocol being run on the network. Each network layer protocol has a different addressing scheme. They are usually hierarchical and define networks first and then host or devices on that network. An example of a network address is an IP address, which is a 32-bit address often expressed in decimal format. 192.168.0.1 is an example of an IP address in decimal format.
How do routers function at the network layer of the OSI model?
Routers learn, record, and maintain awareness of different networks. They decide the best path to these networks and maintain this information in a routing table. The routing table includes the following:
Network addresses, which are protocol-specific. If you are running more than one protocol, you have a network address for each protocol.
The interface the router uses to route a packet to a different network.
A metric, which is the distance to a remote network or the weight of the bandwidth, load, delay, and reliability of the path to the remote network.
Routers create broadcast domains. One interface on a router creates a single broadcast domain and collision domain. However, an interface on a switch creates only a single collision domain.
In addition to learning the remote network and providing a path to the network, what other functions do routers carry out?
Routers perform these tasks:
Routers, by default, do not forward broadcasts or multicasts.
Routers can perform bridging and routing functions.
If a router has multiple paths to a destination, it can determine the best path to the destination.
Routers forward traffic based on Layer 3 destination addresses.
Routers can connect Virtual LANs (VLANs).
Routers can provide quality of service for specified types of network traffic.
What is the responsibility of the data link layer (Layer 2)?
The data link layer provides functional and procedural means for connectionless mode among network entities, and for connection mode entities it provides the establishment, maintenance, and release of data link connections among network entities and for the transfer of data link service data units. The data link layer translates messages from the network layer into bits for the physical layer, and it enables the network layer to control the interconnection of data circuits within the physical layer. Its specifications define different network and protocol characteristics, including physical addressing, error notification, network topology, and sequencing of frames. Data link protocols provide the delivery across individual links and are concerned with the different media types, such as 802.2 and 802.3. The data link layer is responsible for putting 1s and 0s into a logical group. These 1s and 0s are then put on the physical wire. Some examples of data link layer implementations are IEEE 802.2/802.3, IEEE 802.5/802.2, packet trailer (for Ethernet, the FCS or CRC), FFDI, HDLC, and Frame Relay.
The IEEE defines what two sublayers of the data link layer?
The two sublayers of the data link layer are
The Logical Link Control (LLC) sublayer
The Media Access Control (MAC) sublayer
These two sublayers provide physical media independence.
For what is the LLC sublayer responsible?
The Logical Link Control (802.2) sublayer is responsible for identifying different network layer protocols and then encapsulating them to be transferred across the network. An LLC header tells the data link layer what to do with a packet after it is received.
What functions does the Media Access Control (MAC) sublayer provide?
The MAC sublayer specifies how data is placed and transported over the physical wire. The LLC layer communicates with the network layer, but the MAC layer communicates downward directly to the physical layer. Physical addressing (MAC addresses), network topologies, error notification, and delivery of frames are defined at this sublayer.
What are some network devices that operate at the data link layer?
Bridges and switches are network devices that operate at the data link layer. Both devices filter traffic by MAC addresses.
What is the function of the OSI model's physical layer (Layer 1)? Give some examples of physical layer implementations.
The physical layer defines the physical medium. It defines the media type, the connector type, and the signaling type (baseband versus broadband). This includes voltage levels, physical data rates, and maximum cable lengths. The physical layer is responsible for converting frames into electronic bits of data, which are then sent or received across the physical medium. Twisted pair, coaxial cable, and fiber-optic cable operate at this level. Other implementations at this layer are repeaters/hubs, RJ-45.
The Ethernet and IEEE 802.3 standards define what three physical wiring standards that operate at 10 Mbps?
These physical wiring standards operate at 10 Mbps:
10Base2
10Base5
10BaseT
What are collision domains?
In Ethernet segments, devices connect to the same physical medium. Because of this, all devices receive all signals sent across the wire. If two devices send a packet at the same time, a collision occurs. In the event of a collision, the two devices run a backoff algorithm and resend the packet. The devices retransmit up to 15 times. The first station to detect a collision issues a jam signal. When a jam signal is sent from a workstation, it affects all of the machines on the segment, not just the two that collided; when the jam signal is on the wire, no workstations can transmit data. The more collisions that occur in a network, the slower it will be, because the devices will have to resend the packet. A collision domain defines a group of devices connected to the same physical medium.
What are broadcast domains?
A broadcast domain defines a group of devices that receive each others' broadcast messages. As with collisions, the more broadcasts that occur on the network, the slower your network will be. This is because every device that receives a broadcast must process it to see if the broadcast is intended for it.
What devices are used to break up collision and broadcast domains?
Switches and bridges are used to break up collision domains. They create more collision domains and fewer collisions. Routers are used to break up broadcast domains. They create more broadcast domains and smaller broadcast areas.
How do the different layers of the OSI model communicate with each other?
Each layer of the OSI model can communicate only with the layer above it, below it, and parallel to it (a peer layer). For example, the presentation layer can communicate with only the application layer, session layer, and presentation layer on the machine it is communicating with. These layers communicate with each other using protocol data units (PDUs). These PDUs control information that is added to the user data at each layer of the model. This information resides in fields called headers (the front of the data field) and trailers (the end of the data field).
What is data encapsulation?
A PDU can include different information as it goes up or down the OSI model. It is given a different name according to the information it is carrying (the layer it is at). When the transport layer receives upper layer data, it adds a TCP header to the data; this is called a segment. The segment is then passed to the network layer, and an IP header is added; thus, the data becomes a packet. The packet is passed to the data link layer, thus becoming a frame. This frame is then converted into bits and is passed across the network medium. This is data encapsulationApplication layer -- Data
Transport layer -- Segment
Network layer -- Packet
Data link layer -- Frame
There is also the Physical Layer -- Bits
What is the difference between a routing protocol and a routed protocol?
Routing protocols determine how to route traffic to the best location of a routed protocol. Examples of routing protocols are RIP, EIGRP, OSFP, and BGP. Examples of routed protocols are IP and IPX.
What 3 devices are used to segment a LAN?
Router
Switch
Bridge
Quick Notes - CABLING TECHNOLOGY
What is a straight-through cable, and when would you use it?
A straight-through cable is the same at both ends. A straight-through cable uses pins 1, 2, 3, and 6. The send and receive wires are not crossed. You should use a straight-through cable when connecting dissimilar devices. Examples include connecting PCs to switches or hubs or a router to a switch or a hub.
What is a crossover cable, and when would you use it?
A crossover cable is a cable that has the send and receive wires crossed at one of the ends. On a Category 5 cable, the 1 and 3 wires and the 2 and 6 wires are switched on one of the cable's ends. You should use a crossover cable when connecting similar devices, such as connecting a router to a router, a switch to a switch or hub, a hub to a hub, or a PC to a PC.
Important tip -- Router (think of it as a PC) to PC via 10BaseT (NIC) uses a "crossover cable". (contradicts the rule)
How do you set up a console session to a Cisco device?
To set up a console session to a Cisco device, you connect a rollover cable to the console port on the Cisco device. You then connect the other end to your PC and configure a terminal emulation application to the following com settings: 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
What is the maximum cable length for each of the following?
The maximum cable lengths are as follows:
10Base2 (thinnet) 185 meters
10Base5 (thicknet) 500 meters
10BaseT 100 meters
10BaseFL 2000 meters (400 meters in a shared environment and 2000 meters in a point-to-point environment)
100BaseT 100 meters
What does Base stand for in 10BaseT and 100BaseT?
Base in 10BaseT and 100BaseT stands for baseband. Baseband is a network technology in which only one carrier frequency (signal) is used.
What is the difference between baseband and broadband?
Baseband is a network technology in which only one carrier frequency is used (such as Ethernet). Broadband is a network technology in which several independent channels are multiplexed into one cable (for example, a T1 line).
Quick Notes - ACCESS LISTS
Besides named access lists, what are the two types of IP access lists?
The two types of IP access lists are standard and extended.
What criteria do standard IP access lists use to filter packets?
Standard IP access lists filter packets by the source address. This results in the packet's being permitted or denied for the entire protocol suite based on the source network IP address.
What criteria do extended IP access lists use to filter packets?
Extended IP access lists filter packets by source address, destination address, protocols, and port numbers.
In what two ways can IP access lists be applied to an interface?
Access lists can be applied as inbound or outbound access lists. Inbound access lists process packets as they enter a router's interface and before they are routed. Outbound access lists process packets as they exit a router's interface and after they are routed.
How many access lists can be applied to an interface on a Cisco router?
Only one access list per protocol, per direction, per interface can be applied on a Cisco router. Multiple access lists are permitted per interface, but they must be for a different protocol.
How are access lists processed?
Access lists are processed in sequential, logical order, evaluating packets from the top down, one statement at a time. As soon as a match is made, the permit or deny option is applied, and the packet is not applied to any more access list statements. Because of this, the order of the statements within any access list is significant.
What is at the end of each access list?
At the end of each access list, an implicit deny statement denies any packet not filtered in the access list.
What are the number ranges used to define standard and extended IP access lists?
The number ranges used to define standard and extended IP access lists are as follows:
• Standard IP access lists 1 to 99 and 1300 to 1999• Extended IP access lists 100 to 199 and 2000 to 2699
When implementing access lists, what are wildcard masks?
Wildcard masks define the subset of the 32 bits in the IP address that must be matched. Wildcards are used with access lists to specify a host, network, or part of a network. Wildcard masks work exactly the opposite of subnet masks. In subnet masks, 1 bits are matched to the network portion of the address, and 0s are wildcards that specify the host range. In wildcard masks, when 0s are present, the octet address must match.
What is the IOS command syntax used to create a standard IP access list?
Here is the command syntax to create a standard IP access list:
access-list access-list-number {permit deny} source-address [wildcard mask]access-list-number is a number from 1 to 99.
For example:
RouterA(config)#access-list 10 deny 192.168.0.0 0.0.0.255
How can you display all access lists on a Cisco router?
To display all access lists on a Cisco router, use the show access-list command: RouterA#show access-list Standard IP access list 10 deny 192.168.0.0, wildcard bits 0.0.0.255Extended IP access list 101 permit tcp any any eq www permit udp any any eq domain permit udp any eq domain any permit icmp any any deny tcp 192.168.10.0 0.0.0.255 any eq wwwRouterA#
Quick Notes - FRAME RELAY
What protocol does Frame Relay rely on for error checking?
Frame Relay does not rely on any certain protocol for error checking. Instead, it relies on upper-layer protocols to provide error checking. For example, Frame Relay relies on TCP to provide error checking in an IP network.
At what layers of the OSI model does Frame Relay operate?
Frame Relay operates at the two lower layers of the OSI model (data link and physical).
What is the difference between switched virtual circuits (SVCs) and permanent virtual circuits (PVCs)?
SVCs are virtual circuits that are dynamically established when data needs to be transferred and that are terminated when data transmission is complete. SVCs consist of four states: call setup, data transfer, idle, and call termination. PVCs are permanently established virtual circuits that operate in one of two states: idle or data transfer. When the PVC is idle, the connection between the DTE devices is still active.
What is a Data Link Connection Identifier (DLCI)?
A DLCI is a number that identifies the logical circuit between the router and the Frame Relay switch. It is the Frame Relay Layer 2 address. The Frame Relay switch maps DLCIs between each pair of routers to create a PVC. For IP devices at the end of each virtual circuit to communicate, their IP addresses need to be mapped to DLCIs. If you are running Cisco IOS 11.2 or later, mapping is done automatically using Inverse ARP. DLCIs have local significance. Think of DLCIs as the MAC address of the Frame Relay network.
What is the committed information rate (CIR)?
The CIR is the committed information rate, by the service provider, in bits per second, at which data will be transferred. The service provider sends any data in excess of this rate if its network has capacity at that time.
How does Frame Relay use Inverse ARP?
Frame Relay uses Inverse ARP as a way to dynamically map a network layer address to a DLCI. With Inverse ARP, the router can discover the network address of a device associated with a VC.
What is the Local Management Interface (LMI)?
The LMI is a signaling standard between a CPE device (a router) and the Frame Relay switch that is responsible for managing and maintaining status between the devices. It is autosensed with Cisco IOS Release 11.2 and later.
In Frame Relay, what is Forward Explicit Congestion Notification (FECN)?
The FECN is the bit in the Frame Relay header that signals to anyone receiving the frame (switches and DTEs) that congestion is occurring in the same direction as the frame. Switches and DTEs can react by slowing the rate at which data is sent in that direction.
What is Backward Explicit Congestion Notification (BECN)?
The BECN is the bit in the Frame Relay header that signals to switches and DTEs receiving the frame that congestion is occurring in the direction opposite (backward) that of the frame. If switches and DTE devices detect that the BECN bit in the Frame Relay header is set to 1, they slow the rate at which data is sent in that direction.
In the Frame Relay header, what is the discard eligibility (DE) bit?
If congestion is detected on the Frame Relay network, the DE bit is turned on in the Frame Relay header. The DE bit is turned on for frames that are in excess of the CIR. The DE bit tells a switch which frames to discard if they must be discarded.
What is the default LMI type for Cisco routers that are configured for Frame Relay?
The default LMI for Cisco routers configured for Frame Relay is Cisco. If you are running Cisco IOS Release 11.2 or later, the Cisco router tries to autosense which LMI type the Frame Relay switch is using. If it cannot autosense the LMI type, the router uses Cisco as its LMI type. The three types of LMIs supported by Cisco routers are:
Cisco
ANSI
Q933a
When a router receives LMI information, it updates its VC status to one of three states. What are these three states?
The three states that a VC uses to update its status are as follows:
Active state The connection is active, and routers can exchange data.
Inactive state The local connection to the Frame Relay switch is working, but the remote router's connection to the Frame Relay switch is not working.
Deleted state Indicates that no LMIs are being received from the Frame Relay switch or that there is no service between the router and the Frame Relay switch.
How do you enable Frame Relay on a Cisco router?
To enable Frame Relay on a Cisco router, you must first enable the serial interface for Frame Relay encapsulation with the encapsulation frame-relay interface command:
RouterB(config)#int s 0
RouterB(config-if)#ip address 192.168.1.1 255.255.255.0
RouterB(config-if)#encapsulation frame-relay
The default encapsulation for a serial interface configured for Frame Relay is cisco. If you are connecting to a non-Cisco router, how do you change the encapsulation type?
If you are connecting to a non-Cisco router in a Frame Relay network, you need to specify ietf as the encapsulation type:
RouterB(config-if)#ip address 192.168.1.1 255.255.255.0
RouterB(config-if)#encapsulation frame-relay ietf
If you are using Cisco IOS Release 11.1 or earlier, or if you do not want to autosense the LMI type, how do you define the LMI type on a Cisco router?
To define the LMI type on a Cisco router, use the frame-relay lmi-type {ansi cisco q933a} interface command:
RouterB(config-if)#ip address 192.168.1.1 255.255.255.0
RouterB(config-if)#encapsulation frame-relay
RouterB(config-if)#frame-relay lmi-type ansi
If Inverse ARP is disabled on your router, how do you reenable it?
Inverse ARP is enabled by default on a Cisco router. If it is disabled, reenable it by using the following command:
RouterB(config-if)#frame-relay inverse-arp [protocol] [dlci]
Supported protocols indicated by the protocol option include ip, ipx, decnet, appletalk, vines, and xns.
If a remote router does not support Inverse ARP, you must define the address-to-DLCI table
statically. How do you create these static maps?
To define static maps on a Cisco router, use the following command:
RouterA(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf cisco] [payload-compress packet-by-packet]
where:
• protocol defines the supported protocol bridging or LLC.
• protocol-address is the remote router's network layer address.
• dlci defines the remote router's local DLCI.
• broadcast specifies whether you want to forward broadcasts over the VC, permitting dynamic routing protocols over the VC.• ietf cisco is the encapsulation type.
What Cisco IOS command displays the LMI traffic statistics and LMI type?
The show frame-relay lmi command displays the LMI traffic statistics and LMI type:
RouterA#show frame-relay lmi
LMI Statistics for interface Serial0 (Frame Relay DTE)
LMI TYPE = CISCO Invalid Unnumbered info 0
Invalid Prot Disc 0 Invalid dummy Call Ref 0
Invalid Msg Type 0 Invalid Status Message 0
Invalid Lock Shift 0 Invalid Information ID 0
Invalid Report IE Len 0 Invalid Report Request 0
Invalid Keep IE Len 0 Num Status Enq. Rcvd 1748
Num Status msgs Sent 1748 Num Update Status Sent 0
Num St Enq. Timeouts 0
routera#
How do you display the current Frame Relay map entries and information about these connections on a Cisco router?
To view the current map entries and information about the connections, use the show frame-relay map command:
RouterA#show frame-relay map
Quick Notes - ROUTING
How do OSPF-enabled routers build adjacencies and exchange their routing tables?
OSPF-enabled routers build adjacencies by sending Hello packets out through all OSPF-enabled interfaces.
If these routers share a command link and agree on parameters set within their Hello packets then they become neighbors. If these parameters differ then the routers do not become neighbors and communication stops.
OSPF routers form adjacencies with certain routers. These routers are determined by the layer 2 (data link) media type and as soon as the adjacencies are formed each router sends LSAs (Link State Advertisements) to all adjacent routers. The LSAs describe the state of each router's links. There are multiple LSA types and a route that receives an LSA from a neighbor records the LSA in a link-state database and floods a copy of the LSA to all its other neighbors.
When all databases are complete - then each router uses the SPF (Shortest-Path First) algorithm to calculate a loop-free topology and builds its routing table based on this topology.
It is important to note that the Hello protocol is bidrectional and is the means by which neighbors are discovered and acts as keepalives between neighboring routers. It also establishes and maintain neighbor relationships and elects the DR (Designated Router) and BDR (Backup Designated Router) to represent the segment on Broadcast and NBMA (nobroadcast multiaccess) networks.
Note: Hello protocols are sent periodically sent out each OSPF-enabled interface using IP multicast address 224.0.0.5. The default interval on NBMA (nonbroadcast multiaccess) networks is 30 seconds. The default interval on Broadcast, Point-to-point, and point-to-multipoint networks is 10 seconds.
What are LSAs (link-state advertisements)?
LSAs are sent out all OSPF-enabled router interfaces describing the state of the router's links. They are also packets that OSPF uses to advertise changes in the condition of a link or other OSPF routers.
Name two LSA (link-state advertisement) types?
Type 1 LSAs are router LSAs and are generated by each router for the area to which the router belongs. These LSAs describe the states of the router'links to the area (area 0 for example) and are flooded within a single area (area 0 for example).
Type 2 LSAs are network LSAs and are generated by the DR (Designated Router) and the BDR (Backup Designated Router). They describe the routers attached to a particular network and are flooded within a single area (area 0 for example).
What is the routing metric OSPF is based on?
Bandwith.
Formula: Cost = 100,000,000 / bandwidth in bits per seconds
The cost of a 100 MBbps connection would be:
1000,000,000 / 100,000,000 =1
Based on the schema above -- if adjacencies are established with only with the DR (Designated Router) and BDR (Backup Designated Router)- what is the circuit count?
Formula:
2(n - 1) where n is the number of routers in the network.
2(5 - 1) = 8 circuits.
A circuit can also be thought of as an adjacency or connection.
Count four going into the DR and 4 going into the BDR for a total of 8.
Note: OSPF avoids synchronizing between every pair of routers in the network by using a DR and BDR. This way adjacencies are formed only to the DR and BDR, and the number of LSAs sent over the network is reduced. Now only the DR and BDR have four adjacencies, and all the other routers have two.
On an OSPF-enabled router -- what is the router ID and where does an OSPF-enabled router receive its router ID?
To initialize - OSPF must be able to define a router ID. The most common and stable source for a router ID is the IP address set on the logical loopback interface that is always available. If no logical interface is defined -- then the router receives its ID from the highest IP address on the physical interfaces.
Note: If two loopback addresses are defined -- it will use the highest loopback address. Think highest logical or highest physical interface address.
Name five OSPF network types:
Broadcast networks: Ethernet/Token Ring. OSPF-enabled routers on broadcast networks elect a DR (Designated Router) and BDR (Backup Designated Router). All the routers in the network form adjacencies with the DR and BDR. Note: OSPF packets are multicast to the DR and BDR.
NBMA (nonbroadcast multiaccess) networks: Frame Relay/X.25/ATM. NBMA networks can connect more than two routers but have no broadcast functionality. These networks elect an DR and BDR. Note: OSPF packets are unicast.
Point-to-point networks: A physical DS1 (T1) for example.
Point-to-point networks connect a pair of routers and always becomes adjacent.
Point-to-multipoint networks: Point-to-multipoint networks are a special configuration of NBMA networks in which networks are treated as a collection of point-to-point links. Point-to-multipoint networks do not elect a DR or BDR. Note: OSPF packets are multicast.
Virtual links: Virtual links area special configuration that the router interprets as unnumbered point-to-point networks. The network administrator creates/defines virtual links.
What is routing?
Routing is the process in which items are forwarded from one location to another. Routing is a hop-by-hop paradigm.
A Cisco router performs routing and switching functions. Describe what each function does.
Routing is a way to learn and maintain awareness of the network topology. Each router maintains a routing table in which it looks up the destination Layer 3 address to get the packet one step closer to its destination.
The switching function is the actual movement of temporary traffic through the router, from an inbound interface to an outbound interface.
What are the three types of routes you can use in a Cisco router?
The three types of routes are static routes, dynamic routes, and default routes.
What is the difference between static and dynamic routes?
Static routes are routes that an administrator manually enters into a router. Dynamic routes are routes that a router learns automatically through a routing protocol.
What is a default route?
Also known as the gateway of last resort, a default route is a special type of static route with an all-zeros network and network mask. The default route is used to route any packets to a network that a router does not directly know about to a next-hop router. By default, if a router receives a packet to a destination network that is not in its routing table, it drops the packet. When a default route is specified, the router does not drop the packet. Instead, it forwards the packet to the IP address specified in the default route.
What is a routing protocol?
A routing protocol defines the set of rules used by a router when it communicates with neighboring routers. Routing protocols listens for packets from other participants in order to learn and maintain a routing table.
What is administrative distance?
Administrative distance (AD) is an integer from 0 to 255 that rates the trustworthiness of routing information received on a router from a neighboring router. The AD is used as the tiebreaker when a router has multiple paths from different routing protocols to the same destination. The path with the lower AD is the one given priority.
What are the three classes of routing protocols?
1) Distance vector 2) Link-state 3) Balanced hybrid
What is the AD for each of the following?
Directly connected interface 0
Static route 1
EIGRP 90
IGRP 100
OSPF 110
RIP 120
External EIGRP 170
Unknown 255
How do distance vector routing protocols function?
Also known as Bellman-Ford-Fulkerson algorithms, distance vector routing protocols pass complete routing tables to neighboring routers. Neighboring routers then combine the received routing table with their own routing table. Each router receives a routing table from its directly connected neighbor. Distance vector routing tables include information about the total cost and the logical address of the first router on the path to each network they know about.
How do distance vector routing protocols keep track of any changes to the internetwork?
Distance vector routing protocols keep track of an internetwork by periodically broadcasting updates out all active interfaces. This broadcast contains the entire routing table. This method is often called routing by rumor.
Slow convergence of distance vector routing protocols can cause inconsistent routing tables and routing loops.
What are some mechanisms that distance vector protocols implement to prevent routing loops and inconsistent routing tables?
Here are some of the ways distance vector routing protocols prevent routing loops and inconsistent routing tables:
• Maximum hop count• Split horizon
• Route poisoning• Holddowns
What is split horizon?
The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original update came.
What is convergence?
Convergence is when all routers have consistent knowledge and correct routing tables.
What is route poisoning?
With route poisoning, when a distance vector routing protocol notices that a route is no longer valid, the route is advertised with an infinite metric, signifying that the route is bad. In RIP, a metric of 16 is used to signify infinity. Route poisoning is used with holddowns.
What are hold-down timers?
Hold-down timers prevent regular update messages from reinstating a route that might have gone bad. Hold-down timers also tell routers to hold for a period of time any changes that might affect routes.
What are triggered updates?
When a router notices that a directly connected subnet has changed state, it immediately sends another routing update out its other interfaces rather than waiting for the routing update timer to expire. Triggered updates are also known as Flash updates.
What is IP RIP?
IP RIP is a true distance vector routing protocol that sends its complete routing table out all active interfaces every 30 seconds. IP RIP uses a hop count as its metric to determine the best path to a remote network. The maximum allowable hop count is 15, meaning that 16 is unreachable. There are two versions of RIP. Version 1 is classful, and version 2 is classless. IP RIP can load-balance over as many as six equal-cost paths.
What four timers does IP RIP use to regulate its performance?
Here are the four timers that IP RIP uses to regulate its performance: • Route update timer Time between router updates. The default is 30 seconds.• Route invalid timer Time that must expire before a route becomes invalid. The default is 180 seconds.• Route hold-down timer If IP RIP receives an update with a hop count higher than the metric recorded in the routing table, the router goes into holddown for 180 seconds.• Route flush timer Time from when a route becomes invalid to when it is removed from the routing table. The default is 240 seconds.
How do you stop RIP updates from propagating out an interface on a router?
Sometimes you do not want RIP updates to propagate across the WAN, wasting valuable bandwidth or giving out valuable information about your internetwork. The easiest way to stop RIP updates from propagating out an interface is to use the passive-interface global configuration command.
How do you display the contents of a Cisco IP routing table?
The show ip route command displays the Cisco routing table's contents.
What is Interior Gateway Routing Protocol (IGRP)?
IGRP is a Cisco proprietary distance vector routing protocol. IGRP has a default hop count of 100 hops, with a maximum hop count of 255. IGRP uses bandwidth and line delay as its default metric, but it can also use reliability, load, and MTU.
How do you enable IGRP on a Cisco router?
The way you enable IGRP on a Cisco router is similar to the way you enable RIP, except you specify IGRP as the protocol and add an autonomous system number. For example:
RouterA(config)#router igrp 10 (10 is the AS number)
RouterA(config-router)#network 192.168.0.0
RouterA(config-router)#network 192.168.1.0
RouterA(config-router)#network 192.168.2.0
What four timers does IGRP use to regulate its performance?
The four timers IGRP uses to regulate its performance are as follows:
• Route update timer Time between router updates The default is 90 seconds.
• Route invalid timer Time that must expire before a route becomes invalid . The default is 270 seconds.
• Route hold-down timer If a destination becomes unreachable, or if the next-hop router increases the metric recording in the routing table, the router goes into holddown for 280 seconds.
• Route flush timer[md]Time from when a route becomes invalid to when it is removed from the routing table. The default is 630 seconds.
Quick Notes - SWITCHING
What are three types of LAN traffic?
Unicasts - intended for one host.
Broadcasts - intended for everyone.
Multicasts - intended for a only a subset or group within an entire network.
What are unicast frames?
Unicast frames are the most common type of network traffic. A unicast frame is a frame intended for only one host. The only station that processes this frame is one station that has its own MAC address in the destination portion of packet.
What are broadcast frames?
Broadcast frames are frames intended for everyone. Stations view broadcast frames as public service announcements. All stations receive and process broadcast frames. In large networks, broadcasts can cause serious performance degradation in network hosts - (broadcast storm).
The destination address of broadcast frames (Data Link / Layer 2 broadcast addresses is FF-FF-FF-FF-FF-FF or alternatively all 1s in binary (11111111).
What are multicast frames?
Multicast frames address a group of devices that have a common interest. These frames allow the source to send only one copy of the frame on the network even though it is intended for several stations. Only stations that have a card that is configured by software to receive multicast frames for a particular multicast group can process a frame to that multicast address - all other stations discard multicast frames
What three major functions do Data Link Layer / Layer 2 Switches perform?
Address learning
Packet forwarding/filtering
Loop avoidance by spanning tree
What will occur when you attempt to segment a network with hubs and repeaters?
Basically, hubs and repeaters become extensions of the physical cable plant. All devices that connect to either a hub or a repeater share the same bandwidth and by definition hubs and repeaters create a single broadcast and collision domain.
Think of both devices are pass-through devices much like a electrical power-strip. Hubs and repeaters reside on the Physical Layer / Layer 1 of the OSI model where they pass 0s and 1s along the wire or up to the Data Link Layer. CSUs / Channel Service Units fall into the same category. All are regarded as unintelligent devices. No addressing takes place on the Physical layer.
What three devices are used to segment a LAN?
Router - logical addressing - IP address
Switch - physical addressing - MAC address
Bridge - physical addressing - MAC addres
What is microsegmentation?
Each workstation or network device on the network has its own dedicated segment - also known as a Collision Domain - to a switch. Each device gets the segments full bandwidth and does not have to share the dedicated segment with other devices. Collisions are reduced because each segment is its own Collision Domain.
Important: Full-duplex transmission is achieved by microsegmentation. Each device can send and receive at the same time which doubles the amount of bandwidth between nodes.
What are the three switching methods in Cisco Catalyst switches?
The three frame operating modes to handle frame switching are
Store-and-forward
Cut-through
Fragment-free
What is the Cisco Catalyst store-and-forward switching method?
In the store-and-forward switching method, the switch receives the entire frame before it forwards it. The switch reads the cyclic redundancy check (CRC) to make sure the frame is not bad. If the frame is good, the switch forwards it. Because the switch stores the frame before forwarding it, latency is introduced in the switch. Latency through the switch varies with the size of the frame.
What is the Cisco Catalyst cut-through switching method?
In cut-through switching mode, the switch only checks the frame's destination address and immediately begins forwarding the frame out the appropriate port. Because the switch checks the destination address in only the header and not the entire frame, the switch forwards a collision frame or a frame that has a bad CRC.
What is the Cisco Catalyst fragment-free switching method?
Also known as modified cut-through, fragment-free switching checks the first 64 bytes before forwarding the frame. Ethernet specifications state that collisions should be detected during the first 64 bytes of the frame. By reading the first 64 bytes of the frame, the switch can filter most collisions, although late collisions are still possible.
What is the default switching mode in Cisco Catalyst 1900 switches?
The default switching mode for the Catalyst 1900 is fragment-free.
What is half-duplex transmission mode?
Half-duplex transmission is the default mode that Ethernet functions in. In half-duplex transmission, a device can only send or receive--not do both at once. In half-duplex mode, stations are susceptible to collisions, and efficiency is rated at 50 to 60 percent.
What is full-duplex transmission mode?
In full-duplex mode, a station can send and receive at the same time. In full-duplex mode, collision detection is disabled. This mode offers 100 percent efficiency in both directions.
On a Cisco Catalyst 1900 switch, what are the default duplex settings for 10BaseT/100BaseT ports, default switching mode, and default protocols?
What are dynamic addresses on a Catalyst switch?
Dynamic addresses are addresses that the switch learns about dynamically through the learning process. If the switch does not see a MAC address for a certain amount of time, it drops the MAC address.
What are permanent MAC addresses on a Catalyst switch?
Permanent MAC addresses are entered manually by the administrator and are not aged out.
What is carrier sense multiple access collision detect (CSMA/CD)?
CSMA/CD describes the Ethernet access method. In CSMA/CD, many stations can transmit on the same cable, and no station has priority over any other. Before a station transmits, it listens on the wire to make sure no other station is transmitting. If no other station is transmitting, the station transmits across the wire. CSMA/CD is all about devices taking turns using the wire.
What are MAC addresses?
For computers to identify each other on the data link layer, they need a MAC address (hardware address). All devices on a LAN must have a unique MAC address. A MAC address is a 48-bit (six octet) address burned into a network interface card. The first three octets (24 bits) of the MAC address indicate the vendor that manufactured the card. This is called the Organization Unique Identifier (OUI). The last three octets of the MAC address are the unique host address. An example of a MAC address is 00-80-C6-E7-9C-EF.
What are the three types of LAN traffic?
The three types of LAN traffic are:
Unicasts
Broadcasts
Multicasts
What are unicast frames?
Unicast frames are the most common type of LAN traffic. A unicast frame is a frame intended for only one host. In unicast frames, the only station that processes the frame is the station that has its own MAC address in the destination portion of the packet.
What are broadcast frames?
Broadcast frames are frames intended for everyone. Stations view broadcast frames as public service announcements. All stations receive and process broadcast frames. In large networks, broadcasts can bring the network to a crawl, because every computer must process them.
What is the destination address of broadcast frames?
The destination address of broadcast frames (Layer 2 broadcast addresses) is FF-FF-FF-FF-FF-FF, or all 1s in binary.
What are multicast frames?
Multicast frames address a group of devices that have a common interest. These frames allow the source to send only one copy of the frame on the network even though it is intended for several stations. Only stations that have a card that is configured to receive multicast frames process them. All other stations discard multicast frames.
What devices can you use to segment a LAN at Layer 1, Layer 2, and Layer 3?
Three devices you can use to segment a LAN are:
Hubs/repeaters (Layer 1)
Bridges/switches (Layer 2) - physical addresses
Routers (Layer 3) - logical addresses
What happens when you segment the network with hubs/repeaters?
Because hubs and repeaters operate at the physical layer of the OSI model, segmenting a network with these devices appears as an extension to the physical cable. Hubs and repeaters are transparent to devices. They are unintelligent devices. All devices that connect to a hub/repeater share the same bandwidth. Hubs/repeaters create a single broadcast and collision domain.
What is the advantage of segmenting a network with bridges/switches?
Bridges/switches operate at Layer 2 of the OSI model and filter by MAC address. Each port on a bridge/switch provides full-dedicated bandwidth and creates a single collision domain. Because bridges/switches operate at Layer 2 of the OSI model, they cannot filter broadcasts, and they create a single broadcast domain. For the CCNA test, remember that switches create more collision domains and fewer collisions.
What is the difference between bridges and switches?
Bridges and switches function the same way; the only difference is in how they are implemented. Bridges are implemented by software and usually have a couple of network ports. Switches are implemented in hardware by ASIC chips and have many ports.
What are the advantages and disadvantages of segmenting the LAN with routers?
An advantage of segmenting the LAN with routers is that each interface on a router creates a single broadcast and collision domain. Routers operate at Layer 3 of the OSI model and do not propagate broadcasts. Some disadvantages are that routers are not transparent and are implemented in software, thus introducing latency in the network.
What is the Maximum Transmission Unit (MTU) for an Ethernet frame?
1500 bytes is the MTU for an Ethernet frame. You will notice that some publications state that the MTU for Ethernet is 1518 bytes. This is correct also. But what is the true answer? The MTU for Ethernet, including the header, source and destination address, data, and CRC is 1518 bytes. The MTU for the data portion of the frame is 1500 bytes.
What three major functions do Layer 2 switches provide?
The three major functions that Layer 2 switches provide are
Address learning
Packet forwarding/filtering
Loop avoidance by spanning tree
What are some advantages of switches?
Some advantages of switches are as follows:
They increase available network bandwidth.
They reduce the number of users per segment.
They provide dedicated bandwidth to each segment.
Transparent bridging (switching) provides five bridging functions to determine what to do when it receives a frame.
What are these five processes?
The five processes are:
Learning
Flooding
Filtering
Forwarding
Aging
In transparent bridging, what is the learning process?
The first process a bridge goes through when it is powered on is the learning process. The MAC address table on the bridge contains no entries, and the bridge goes through the learning process to record all workstations on every interface. In the learning process, the bridge records the source MAC address and source port number in the MAC address table every time it sees a frame.
In transparent bridging, what is the flooding process?
When a bridge is first turned on, it has no MAC address in its table. When a switch receives a unicast frame, it knows the source address and port from which the unicast frame came, but no entry exists in its table for the destination address. This is called an unknown unicast frame. When a switch receives an unknown unicast frame, it sends the frame out all forwarding interfaces on the bridge except the interface that received the frame. This process is the flooding process.
In transparent bridging, what is the filtering process?
The filtering process occurs when the source and destination addresses reside on the same interface on the bridge. Because the bridge does not need to forward a frame in which the destination and source addresses reside on the same interface, it filters the frame and discards it.
In transparent bridging, what is the forwarding process?
The forwarding process occurs when a switch receives a unicast frame and has an entry of the destination address in its MAC table. The switch then forwards the frame to the interface where that destination address resides.
In transparent bridging, what occurs during the aging process?
Every time a bridge learns a source address, it time-stamps the entry. When the bridge sees a frame from this source, it updates the time stamp. If the bridge does not hear from the source for a specific amount of time (called the aging timer), the bridge deletes the entry from its MAC address table. This process is the aging process.
What is the default aging time in transparent bridges?
The default aging timer is 5 minutes.
What is the Spanning-Tree Protocol (STP)?
STP is a loop-prevention bridge-to-bridge protocol. Its main purpose is to dynamically maintain a loop-free network. It does this by sending out Bridge Protocol Data Units (BPDUs), discovering any loops in the topology, and blocking one or more redundant links.
How does STP maintain a loop-free network?
STP maintains a loop-free network by
Electing a root bridge
Electing a root port on each nonroot bridge
Electing designated ports
Putting in the blocking state any port that is not a root port or designated port
In spanning tree, what is a Bridge ID (BID)?
A BID is an 8-byte field that is composed of the bridge's 6-byte MAC address and a 2-byte bridge priority.
What is the default bridge priority in a Bridge ID for all Cisco switches?
32,768
In spanning tree, what is path cost?
Path cost is a calculation to determine the link's bandwidth. It is a value assigned to each port that is based on the port's speed.
What is the spanning tree path cost for each of the following?
10 Mbps
100 Mbps
1 Gbps
The path costs are as follows:
10 Mbps - 100
100 Mbps - 19
1 Gbps - 4
When calculating a loop-free environment, what four-step decision sequence does spanning tree use to determine what will be the root bridge and which ports will forward or block?
The four-step decision sequence that spanning tree uses to determine the root bridge and which port will forward is as follows:
Step 1. The lowest root BID
Step 2. The lowest path cost to the root bridge
Step 3. The lowest sender BID
Step 4. The lowest port ID
How do bridges pass spanning tree information between themselves?
Bridges pass STP information using special frame called Bridge Protocol Data Units (BPDUs).
How often do bridges send BPDUs out active ports?
The default time that bridges send BPDUs out active ports is 2 seconds.
Note: All ports on a switch listen for BPDUs in case there is a topology change.
In STP, how is a root bridge elected?
In STP, the bridge with the lowest BID is elected the root bridge. All ports on the root bridge are placed in the forwarding state and are called designated ports.
Note: The BID is a 6-byte field that is composed of a default priority (32,768) and a MAC address. Because all Cisco switches use the default priority, the switch with the lowest MAC address is elected the root bridge. As a rule of thumb, lower will always win in spanning tree.
After bridges elect the root bridge, what do they do next?
After electing the root bridge, switches elect root ports. A root port is the port on nonroot bridges that is closest to the root bridge. Every nonroot bridge must select one root port.
How do nonroot bridges decide which port they will elect as a root port?
Nonroot bridges use root path cost to determine which port will be the root port. Root path cost is the cumulative cost of all links to the root bridge. The port with the lowest root path cost is elected the bridge's root port and is placed in the forwarding state.
What is the difference between path cost and root path cost?
Path cost is the value assigned to each port. It is added to BPDUs received on that port to calculate the root path cost. Root path cost is defined as the cumulative cost to the root bridge. In a BPDU, this is the value transmitted in the cost field. In a bridge, this value is calculated by adding the receiving port's path cost to the value contained in the BPDU.
If a nonroot bridge has two redundant ports with the same root path cost, how does the bridge choose which port will be the root port?
If a nonroot bridge has redundant ports with the same root path cost, the deciding factor is the port with the lowest port ID (port number).
After the root bridge and root ports are selected, the last step in spanning tree is to elect designated ports. How do bridges elect designated ports?
In spanning tree, each segment in a bridged network has one designated port. This port is a single port that both sends and receives traffic to and from that segment and the root bridge. All other ports are placed in a blocking state. This ensures that only one port on any segment can send and receive traffic to and from the root bridge, ensuring a loop-free topology. The bridge containing the designated port for a segment is called the designated bridge for that segment. Designated ports are chosen based on cumulative root path cost to the root bridge.
Note: Every active port on the root bridge becomes a designated port.
If a bridge is faced with a tie in electing designated ports, how does it decide which port will be the designated port?
In the event of a tie, STP uses the four-step decision process discussed in Question 30. It first looks for the BPDU with the lowest BID; this is always the root bridge. If the switch is not the root bridge, it moves to the next step: the BPDU with the lowest path cost to the root bridge. If both paths are equal, STP looks for the BPDU with the lowest sender BID. If these are equal, STP uses the link with the lowest port ID as the final tiebreaker.
What are the four spanning tree port states?
The four spanning tree port states are
Blocking
Listening
Learning
Forwarding
Remember that root and designated ports forward traffic and that nondesignated ports block traffic but still listen for BPDUs.
Important note: There is another port state - Disabled - (No frames forwarded, no BPDUs heard). If it shows up in the answer options - select it along with the others.
What is the STP blocking state?
When a switch starts, all ports are in the blocking state. This is to prevent any loops in the network. If there is a better path to the root bridge, the port remains in the blocked state. Ports in the blocked state cannot send or receive traffic, but they can receive BPDUs.
What is the STP listening state?
Ports transition from a blocked state to a listening state. In this state, no user data is passed. The port only listens for BPDUs. After listening for 15 seconds (if the bridge does not find a better path), the port moves to the next state, the learning state.
What is the STP learning state?
In the STP learning state, no user data is being passed. The port quietly builds its bridging table. The default time in the learning state is 15 seconds.
What is the STP forwarding state?
After the default time in the learning state is up, the port moves to the forwarding state. In the forwarding state, the port sends and receives data.
What is STP forward delay?
The forward delay is the time it takes for a port to move from the listening state to the learning state or from the learning state to the forwarding state. The default time is 30 seconds.
What is the hello time in STP timers?
The hello time is the time interval between the sending of BPDUs. The default time is 2 seconds.
What is the Max Age timer?
The Max Age timer is how long a bridge stores a BPDU before discarding it. The default time is 20 seconds (ten missed hello intervals).
What is the default time a port takes to transition from the blocking state to the forwarding state?
The default time a port takes to transition from the blocking state to the forwarding state is 50 seconds: 20 seconds for Max Age, 15 seconds for listening, and 15 seconds for learning.
What does STP do when it detects a topology change in the network due to a bridge or link failure?
If spanning tree detects a change in the network due to a bridge or link failure, at least one bridge interface changes from the blocking state to the forwarding state, or vice versa.
WAN
The three WAN connection types available are leased lines, circuit-switched, and packet-switched. Define the differences between each connection type.
Leased lines are dedicated point-to-point lines that provide a single preestablished WAN communication path from the customer's network to the remote network. Leased lines are usually employed over synchronous connections. They are generally expensive and are always up.
Circuit-switched connections are dedicated for only the duration of the call. The telephone system and ISDN are examples of circuit-switched networks. Packet-switched connections use virtual circuits (VCs) to provide end-to-end connectivity.
Packet-switched connections are similar to leased lines, except that the line is shared by other customers. A packet knows how to reach its destination by programming of switches. Frame Relay is an example of a packet-switched connection.
Define customer premises equipment (CPE), and give an example.
CPE is equipment that is located on the customer's (or subscriber's) premises. It is equipment owned by the customer or equipment leased by the service provider to the customer. An example is your router.
What is the demarcation point (demarc)?
The demarc is the point where the CPE ends and the local loop begins. It is the last responsibility of the service provider and is usually an RJ-45 jack located close to the CPE. Think of the demarc as the boundary between the customer's wiring and the service provider's wiring.
What is the local loop?
The local loop is the physical cable that extends from the demarc to the central office.
Describe the central office (CO).
The CO is the telco switching facility that connects the customer to the provider's switching network. The CO is sometimes referred to as a point of presence. It is the point where the local loop gains access to the service provider's access lines.
What is the toll network?
All the telco switches, COs, and trunk lines inside the WAN provider's network are the toll network.
What are synchronous links?
Synchronous links have identical frequencies and contain individual characters encapsulated in control bits, called start/stop bits, that designate the beginning and end of each character. Synchronous links try to use the same speed as the other end of a serial link.
What are Asynchronous links?
Asynchronous links send digital signals without timing. Asynchronous links agree on the same speed, but there is no check or adjustment of the rates if they are slightly different. Only 1 byte per transfer is sent.
List some typical Layer 2 encapsulation methods for WAN links.
. High-Level Data Link Control (HDLC)
• Point-to-Point Protocol (PPP)
• Serial Line Internet Protocol (SLIP)
• X.25/Link Access Procedure, Balanced (LAPB)
• Frame Relay• Asynchronous Transfer Mode (ATM)
Describe HDLC.
HDLC was derived from Synchronous Data Link Control (SDLC). It is the default encapsulation type on point-to-point dedicated links and circuit-switched connections between Cisco routers. It is an ISO-standard bit-oriented data-link protocol that encapsulates data on synchronous links. HDLC is a connection-oriented protocol that has very little overhead. HDLC lacks a protocol field and therefore cannot encapsulate multiple network layer protocols across the same link. Because of this, each vendor has its own method of identifying the network-layer protocol. Cisco offers a propriety version of HDLC that uses a type field that acts as a protocol field, making it possible for multiple network-layer protocols to share the same link.
By default, Cisco uses HDLC as its default encapsulation method across synchronous lines (point-to-point links). If a serial line uses a different encapsulation protocol, how do you change it back to HDLC?
To change a serial line back to HDLC, use the following interface command on the serial interface you want to change: Router(config-if)#encapsulation hdlc
What is the Point-to-Point Protocol (PPP)?
PPP is an industry-standard protocol that provides router-to-router or router-to-host connections over synchronous and asynchronous links. It can be used to connect to other vendors' equipment. It works with several network-layer protocols, such as IP and IPX. PPP provides authentication through PAP or CHAP.
Describe X.25/LAPB.
X.25/LAPB is an ITU-T standard that has a tremendous amount of overhead because of its strict timeout and windowing techniques. LAPB is the connection-oriented protocol used with X.25. It uses the ABM (Asynchronous Balance Mode) transfer mode. X.25/LAPB was used in the 1980s when WAN links were not as error-free as they are today. X.25 is a predecessor of Frame Relay. X.25 supports both switched and permanent virtual circuits.
What is Frame Relay?
An industry standard, Frame Relay is a switched data link layer protocol that uses virtual circuits to identify the traffic that belongs to certain routers. It provides dynamic bandwidth allocation and congestion control.
Quick Notes - INTERNETWORKING
What are the three layers of the Cisco Hierarchical Model?
The three layers of the Cisco Hierarchical Model are: 1
the access layer
The distribution layer
The core layer
In the Cisco Hierarchical Model, what is the function of the access layer?
Sometimes referred to as the desktop layer, the access layer is the point at which users connect to the network through low-end switches. Some functions of the access layer include:
Connectivity into the distribution layer
Shared Bandwidth
MAC Address filtering (switching)
Segmentation
What is the function of the distribution layer in the Cisco Hierarchical Model?
The distribution layer is also known as the workgroup layer. It is the demarcation point between the access and core layers of the network. Its primary function is to provide routing, filtering, and WAN access. The distribution layer determines how packets access the core, so it is the layer at which to implement policy-based connectivity. Some functions include the following:
Collection point for access layer devices
Broadcast and multicast domain segmentation
Security and filtering services such as firewalls and access lists
Providing translation between different media types
Inter-VLAN routing
What is the role of the core layer in the Cisco Hierarchical Model?
The core layer is the backbone of the network. Its main function is to switch traffic as fast as possible. Therefore, it should not perform any filtering to slow down traffic.
The ISO's OSI Reference Model contains seven layers. What are they? Include the layer number and name of each layer in your answer.
The seven layers of the OSI model are as follows:
Layer 7 - Application layer
Layer 6 - Presentation layer
Layer 5 - Session layer
Layer 4 - Transport layer
Layer 3 - Network layer
Layer 2 - Data link layer
Layer 1 - Physical layer
What are some reasons that the industry uses a layered model?
Here are some reasons why the industry uses a layered model:
It encourages industry standardization by defining what functions occur at each level.
It allows vendors to modify or improve components at only one layer versus rewriting the whole protocol stack.
It helps interoperability by defining standards for the operations at each level.
It helps with troubleshooting.
What does the application layer (Layer 7) of the OSI model do, and what are some examples of this layer?
The application layer is the layer that is closest to the user. This means that this layer interacts directly with the software application. The application layer's main function is to identify and establish communication partners, determine resource availability, and synchronize communication. Some examples include the following:
TCP/IP applications such as Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), WWW, and HTTP.
OSI applications such as Virtual Terminal Protocol, File
Transfer, Access, and Management (FTAM), and Common Management Information Protocol (CMIP).
In the OSI model, what are the responsibilities of the presentation layer (Layer 6)? Give some examples of this layer.
Also known as the translator, the presentation layer provides coding and conversion functions to application layer data. This guarantees that the application layer on another system can read data transferred from the application layer of a different system. Some examples of the presentation layer are:
Compression, decompression, and encryption
JPEG, TIFF, GIFF, PICT, QuickTime, MPEG, and ASCII
What are the functions of the session layer (Layer 5)? Give some examples.
The session layer is responsible for creating, managing, and ending communication sessions between presentation layer entities. These sessions consist of service requests and responses that develop between applications located on different network devices. Some examples include SQL, RPC, NFS, X Window System, ZIP, NetBIOS names, and AppleTalk ASP.
What is the transport layer (Layer 4) responsible for? Give some examples of transport layer implementations.
The transport layer segments and reassembles data from upper-layer applications into data streams. It provides reliable data transmission to upper layers. End-to-end communications, flow control, multiplexing, error detection and correction, and virtual circuit management are typical transport layer functions. Some examples include TCP, UDP*, and SPX.
Note: watch out for end-to-end on communications on the exam! Transport layer.
* Error correction does not apply to UDP - connection-less - unreliable.....
What is flow control, and what are the three methods of implementing it?
Flow control is the method of controlling the rate at which a computer sends data, thus preventing network congestion. The three methods of implementing flow control are
Buffering
Congestion avoidance
Windowing
Almost certain to be on the exam.
Describe the function of the network layer (Layer 3), and give some examples of network layer implementations.
The network layer provides internetwork routing and logical network addresses. It defines how to transport traffic between devices that are not locally attached. The network layer also supports connection-oriented and connectionless service from higher-layer protocols. Routers operate at the network layer. IP, IPX, AppleTalk, and DDP are examples of network layer implementations.
Are network layer addresses physical or logical?
Network layer addresses are logical addresses specific to the network layer protocol being run on the network. Each network layer protocol has a different addressing scheme. They are usually hierarchical and define networks first and then host or devices on that network. An example of a network address is an IP address, which is a 32-bit address often expressed in decimal format. 192.168.0.1 is an example of an IP address in decimal format.
How do routers function at the network layer of the OSI model?
Routers learn, record, and maintain awareness of different networks. They decide the best path to these networks and maintain this information in a routing table. The routing table includes the following:
Network addresses, which are protocol-specific. If you are running more than one protocol, you have a network address for each protocol.
The interface the router uses to route a packet to a different network.
A metric, which is the distance to a remote network or the weight of the bandwidth, load, delay, and reliability of the path to the remote network.
Routers create broadcast domains. One interface on a router creates a single broadcast domain and collision domain. However, an interface on a switch creates only a single collision domain.
In addition to learning the remote network and providing a path to the network, what other functions do routers carry out?
Routers perform these tasks:
Routers, by default, do not forward broadcasts or multicasts.
Routers can perform bridging and routing functions.
If a router has multiple paths to a destination, it can determine the best path to the destination.
Routers forward traffic based on Layer 3 destination addresses.
Routers can connect Virtual LANs (VLANs).
Routers can provide quality of service for specified types of network traffic.
What is the responsibility of the data link layer (Layer 2)?
The data link layer provides functional and procedural means for connectionless mode among network entities, and for connection mode entities it provides the establishment, maintenance, and release of data link connections among network entities and for the transfer of data link service data units. The data link layer translates messages from the network layer into bits for the physical layer, and it enables the network layer to control the interconnection of data circuits within the physical layer. Its specifications define different network and protocol characteristics, including physical addressing, error notification, network topology, and sequencing of frames. Data link protocols provide the delivery across individual links and are concerned with the different media types, such as 802.2 and 802.3. The data link layer is responsible for putting 1s and 0s into a logical group. These 1s and 0s are then put on the physical wire. Some examples of data link layer implementations are IEEE 802.2/802.3, IEEE 802.5/802.2, packet trailer (for Ethernet, the FCS or CRC), FFDI, HDLC, and Frame Relay.
The IEEE defines what two sublayers of the data link layer?
The two sublayers of the data link layer are
The Logical Link Control (LLC) sublayer
The Media Access Control (MAC) sublayer
These two sublayers provide physical media independence.
For what is the LLC sublayer responsible?
The Logical Link Control (802.2) sublayer is responsible for identifying different network layer protocols and then encapsulating them to be transferred across the network. An LLC header tells the data link layer what to do with a packet after it is received.
What functions does the Media Access Control (MAC) sublayer provide?
The MAC sublayer specifies how data is placed and transported over the physical wire. The LLC layer communicates with the network layer, but the MAC layer communicates downward directly to the physical layer. Physical addressing (MAC addresses), network topologies, error notification, and delivery of frames are defined at this sublayer.
What are some network devices that operate at the data link layer?
Bridges and switches are network devices that operate at the data link layer. Both devices filter traffic by MAC addresses.
What is the function of the OSI model's physical layer (Layer 1)? Give some examples of physical layer implementations.
The physical layer defines the physical medium. It defines the media type, the connector type, and the signaling type (baseband versus broadband). This includes voltage levels, physical data rates, and maximum cable lengths. The physical layer is responsible for converting frames into electronic bits of data, which are then sent or received across the physical medium. Twisted pair, coaxial cable, and fiber-optic cable operate at this level. Other implementations at this layer are repeaters/hubs, RJ-45.
The Ethernet and IEEE 802.3 standards define what three physical wiring standards that operate at 10 Mbps?
These physical wiring standards operate at 10 Mbps:
10Base2
10Base5
10BaseT
What are collision domains?
In Ethernet segments, devices connect to the same physical medium. Because of this, all devices receive all signals sent across the wire. If two devices send a packet at the same time, a collision occurs. In the event of a collision, the two devices run a backoff algorithm and resend the packet. The devices retransmit up to 15 times. The first station to detect a collision issues a jam signal. When a jam signal is sent from a workstation, it affects all of the machines on the segment, not just the two that collided; when the jam signal is on the wire, no workstations can transmit data. The more collisions that occur in a network, the slower it will be, because the devices will have to resend the packet. A collision domain defines a group of devices connected to the same physical medium.
What are broadcast domains?
A broadcast domain defines a group of devices that receive each others' broadcast messages. As with collisions, the more broadcasts that occur on the network, the slower your network will be. This is because every device that receives a broadcast must process it to see if the broadcast is intended for it.
What devices are used to break up collision and broadcast domains?
Switches and bridges are used to break up collision domains. They create more collision domains and fewer collisions. Routers are used to break up broadcast domains. They create more broadcast domains and smaller broadcast areas.
How do the different layers of the OSI model communicate with each other?
Each layer of the OSI model can communicate only with the layer above it, below it, and parallel to it (a peer layer). For example, the presentation layer can communicate with only the application layer, session layer, and presentation layer on the machine it is communicating with. These layers communicate with each other using protocol data units (PDUs). These PDUs control information that is added to the user data at each layer of the model. This information resides in fields called headers (the front of the data field) and trailers (the end of the data field).
What is data encapsulation?
A PDU can include different information as it goes up or down the OSI model. It is given a different name according to the information it is carrying (the layer it is at). When the transport layer receives upper layer data, it adds a TCP header to the data; this is called a segment. The segment is then passed to the network layer, and an IP header is added; thus, the data becomes a packet. The packet is passed to the data link layer, thus becoming a frame. This frame is then converted into bits and is passed across the network medium. This is data encapsulationApplication layer -- Data
Transport layer -- Segment
Network layer -- Packet
Data link layer -- Frame
There is also the Physical Layer -- Bits
What is the difference between a routing protocol and a routed protocol?
Routing protocols determine how to route traffic to the best location of a routed protocol. Examples of routing protocols are RIP, EIGRP, OSFP, and BGP. Examples of routed protocols are IP and IPX.
What 3 devices are used to segment a LAN?
Router
Switch
Bridge
Quick Notes - CABLING TECHNOLOGY
What is a straight-through cable, and when would you use it?
A straight-through cable is the same at both ends. A straight-through cable uses pins 1, 2, 3, and 6. The send and receive wires are not crossed. You should use a straight-through cable when connecting dissimilar devices. Examples include connecting PCs to switches or hubs or a router to a switch or a hub.
What is a crossover cable, and when would you use it?
A crossover cable is a cable that has the send and receive wires crossed at one of the ends. On a Category 5 cable, the 1 and 3 wires and the 2 and 6 wires are switched on one of the cable's ends. You should use a crossover cable when connecting similar devices, such as connecting a router to a router, a switch to a switch or hub, a hub to a hub, or a PC to a PC.
Important tip -- Router (think of it as a PC) to PC via 10BaseT (NIC) uses a "crossover cable". (contradicts the rule)
How do you set up a console session to a Cisco device?
To set up a console session to a Cisco device, you connect a rollover cable to the console port on the Cisco device. You then connect the other end to your PC and configure a terminal emulation application to the following com settings: 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
What is the maximum cable length for each of the following?
The maximum cable lengths are as follows:
10Base2 (thinnet) 185 meters
10Base5 (thicknet) 500 meters
10BaseT 100 meters
10BaseFL 2000 meters (400 meters in a shared environment and 2000 meters in a point-to-point environment)
100BaseT 100 meters
What does Base stand for in 10BaseT and 100BaseT?
Base in 10BaseT and 100BaseT stands for baseband. Baseband is a network technology in which only one carrier frequency (signal) is used.
What is the difference between baseband and broadband?
Baseband is a network technology in which only one carrier frequency is used (such as Ethernet). Broadband is a network technology in which several independent channels are multiplexed into one cable (for example, a T1 line).
Quick Notes - ACCESS LISTS
Besides named access lists, what are the two types of IP access lists?
The two types of IP access lists are standard and extended.
What criteria do standard IP access lists use to filter packets?
Standard IP access lists filter packets by the source address. This results in the packet's being permitted or denied for the entire protocol suite based on the source network IP address.
What criteria do extended IP access lists use to filter packets?
Extended IP access lists filter packets by source address, destination address, protocols, and port numbers.
In what two ways can IP access lists be applied to an interface?
Access lists can be applied as inbound or outbound access lists. Inbound access lists process packets as they enter a router's interface and before they are routed. Outbound access lists process packets as they exit a router's interface and after they are routed.
How many access lists can be applied to an interface on a Cisco router?
Only one access list per protocol, per direction, per interface can be applied on a Cisco router. Multiple access lists are permitted per interface, but they must be for a different protocol.
How are access lists processed?
Access lists are processed in sequential, logical order, evaluating packets from the top down, one statement at a time. As soon as a match is made, the permit or deny option is applied, and the packet is not applied to any more access list statements. Because of this, the order of the statements within any access list is significant.
What is at the end of each access list?
At the end of each access list, an implicit deny statement denies any packet not filtered in the access list.
What are the number ranges used to define standard and extended IP access lists?
The number ranges used to define standard and extended IP access lists are as follows:
• Standard IP access lists 1 to 99 and 1300 to 1999• Extended IP access lists 100 to 199 and 2000 to 2699
When implementing access lists, what are wildcard masks?
Wildcard masks define the subset of the 32 bits in the IP address that must be matched. Wildcards are used with access lists to specify a host, network, or part of a network. Wildcard masks work exactly the opposite of subnet masks. In subnet masks, 1 bits are matched to the network portion of the address, and 0s are wildcards that specify the host range. In wildcard masks, when 0s are present, the octet address must match.
What is the IOS command syntax used to create a standard IP access list?
Here is the command syntax to create a standard IP access list:
access-list access-list-number {permit deny} source-address [wildcard mask]access-list-number is a number from 1 to 99.
For example:
RouterA(config)#access-list 10 deny 192.168.0.0 0.0.0.255
How can you display all access lists on a Cisco router?
To display all access lists on a Cisco router, use the show access-list command: RouterA#show access-list Standard IP access list 10 deny 192.168.0.0, wildcard bits 0.0.0.255Extended IP access list 101 permit tcp any any eq www permit udp any any eq domain permit udp any eq domain any permit icmp any any deny tcp 192.168.10.0 0.0.0.255 any eq wwwRouterA#
Quick Notes - FRAME RELAY
What protocol does Frame Relay rely on for error checking?
Frame Relay does not rely on any certain protocol for error checking. Instead, it relies on upper-layer protocols to provide error checking. For example, Frame Relay relies on TCP to provide error checking in an IP network.
At what layers of the OSI model does Frame Relay operate?
Frame Relay operates at the two lower layers of the OSI model (data link and physical).
What is the difference between switched virtual circuits (SVCs) and permanent virtual circuits (PVCs)?
SVCs are virtual circuits that are dynamically established when data needs to be transferred and that are terminated when data transmission is complete. SVCs consist of four states: call setup, data transfer, idle, and call termination. PVCs are permanently established virtual circuits that operate in one of two states: idle or data transfer. When the PVC is idle, the connection between the DTE devices is still active.
What is a Data Link Connection Identifier (DLCI)?
A DLCI is a number that identifies the logical circuit between the router and the Frame Relay switch. It is the Frame Relay Layer 2 address. The Frame Relay switch maps DLCIs between each pair of routers to create a PVC. For IP devices at the end of each virtual circuit to communicate, their IP addresses need to be mapped to DLCIs. If you are running Cisco IOS 11.2 or later, mapping is done automatically using Inverse ARP. DLCIs have local significance. Think of DLCIs as the MAC address of the Frame Relay network.
What is the committed information rate (CIR)?
The CIR is the committed information rate, by the service provider, in bits per second, at which data will be transferred. The service provider sends any data in excess of this rate if its network has capacity at that time.
How does Frame Relay use Inverse ARP?
Frame Relay uses Inverse ARP as a way to dynamically map a network layer address to a DLCI. With Inverse ARP, the router can discover the network address of a device associated with a VC.
What is the Local Management Interface (LMI)?
The LMI is a signaling standard between a CPE device (a router) and the Frame Relay switch that is responsible for managing and maintaining status between the devices. It is autosensed with Cisco IOS Release 11.2 and later.
In Frame Relay, what is Forward Explicit Congestion Notification (FECN)?
The FECN is the bit in the Frame Relay header that signals to anyone receiving the frame (switches and DTEs) that congestion is occurring in the same direction as the frame. Switches and DTEs can react by slowing the rate at which data is sent in that direction.
What is Backward Explicit Congestion Notification (BECN)?
The BECN is the bit in the Frame Relay header that signals to switches and DTEs receiving the frame that congestion is occurring in the direction opposite (backward) that of the frame. If switches and DTE devices detect that the BECN bit in the Frame Relay header is set to 1, they slow the rate at which data is sent in that direction.
In the Frame Relay header, what is the discard eligibility (DE) bit?
If congestion is detected on the Frame Relay network, the DE bit is turned on in the Frame Relay header. The DE bit is turned on for frames that are in excess of the CIR. The DE bit tells a switch which frames to discard if they must be discarded.
What is the default LMI type for Cisco routers that are configured for Frame Relay?
The default LMI for Cisco routers configured for Frame Relay is Cisco. If you are running Cisco IOS Release 11.2 or later, the Cisco router tries to autosense which LMI type the Frame Relay switch is using. If it cannot autosense the LMI type, the router uses Cisco as its LMI type. The three types of LMIs supported by Cisco routers are:
Cisco
ANSI
Q933a
When a router receives LMI information, it updates its VC status to one of three states. What are these three states?
The three states that a VC uses to update its status are as follows:
Active state The connection is active, and routers can exchange data.
Inactive state The local connection to the Frame Relay switch is working, but the remote router's connection to the Frame Relay switch is not working.
Deleted state Indicates that no LMIs are being received from the Frame Relay switch or that there is no service between the router and the Frame Relay switch.
How do you enable Frame Relay on a Cisco router?
To enable Frame Relay on a Cisco router, you must first enable the serial interface for Frame Relay encapsulation with the encapsulation frame-relay interface command:
RouterB(config)#int s 0
RouterB(config-if)#ip address 192.168.1.1 255.255.255.0
RouterB(config-if)#encapsulation frame-relay
The default encapsulation for a serial interface configured for Frame Relay is cisco. If you are connecting to a non-Cisco router, how do you change the encapsulation type?
If you are connecting to a non-Cisco router in a Frame Relay network, you need to specify ietf as the encapsulation type:
RouterB(config-if)#ip address 192.168.1.1 255.255.255.0
RouterB(config-if)#encapsulation frame-relay ietf
If you are using Cisco IOS Release 11.1 or earlier, or if you do not want to autosense the LMI type, how do you define the LMI type on a Cisco router?
To define the LMI type on a Cisco router, use the frame-relay lmi-type {ansi cisco q933a} interface command:
RouterB(config-if)#ip address 192.168.1.1 255.255.255.0
RouterB(config-if)#encapsulation frame-relay
RouterB(config-if)#frame-relay lmi-type ansi
If Inverse ARP is disabled on your router, how do you reenable it?
Inverse ARP is enabled by default on a Cisco router. If it is disabled, reenable it by using the following command:
RouterB(config-if)#frame-relay inverse-arp [protocol] [dlci]
Supported protocols indicated by the protocol option include ip, ipx, decnet, appletalk, vines, and xns.
If a remote router does not support Inverse ARP, you must define the address-to-DLCI table
statically. How do you create these static maps?
To define static maps on a Cisco router, use the following command:
RouterA(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf cisco] [payload-compress packet-by-packet]
where:
• protocol defines the supported protocol bridging or LLC.
• protocol-address is the remote router's network layer address.
• dlci defines the remote router's local DLCI.
• broadcast specifies whether you want to forward broadcasts over the VC, permitting dynamic routing protocols over the VC.• ietf cisco is the encapsulation type.
What Cisco IOS command displays the LMI traffic statistics and LMI type?
The show frame-relay lmi command displays the LMI traffic statistics and LMI type:
RouterA#show frame-relay lmi
LMI Statistics for interface Serial0 (Frame Relay DTE)
LMI TYPE = CISCO Invalid Unnumbered info 0
Invalid Prot Disc 0 Invalid dummy Call Ref 0
Invalid Msg Type 0 Invalid Status Message 0
Invalid Lock Shift 0 Invalid Information ID 0
Invalid Report IE Len 0 Invalid Report Request 0
Invalid Keep IE Len 0 Num Status Enq. Rcvd 1748
Num Status msgs Sent 1748 Num Update Status Sent 0
Num St Enq. Timeouts 0
routera#
How do you display the current Frame Relay map entries and information about these connections on a Cisco router?
To view the current map entries and information about the connections, use the show frame-relay map command:
RouterA#show frame-relay map
Quick Notes - ROUTING
How do OSPF-enabled routers build adjacencies and exchange their routing tables?
OSPF-enabled routers build adjacencies by sending Hello packets out through all OSPF-enabled interfaces.
If these routers share a command link and agree on parameters set within their Hello packets then they become neighbors. If these parameters differ then the routers do not become neighbors and communication stops.
OSPF routers form adjacencies with certain routers. These routers are determined by the layer 2 (data link) media type and as soon as the adjacencies are formed each router sends LSAs (Link State Advertisements) to all adjacent routers. The LSAs describe the state of each router's links. There are multiple LSA types and a route that receives an LSA from a neighbor records the LSA in a link-state database and floods a copy of the LSA to all its other neighbors.
When all databases are complete - then each router uses the SPF (Shortest-Path First) algorithm to calculate a loop-free topology and builds its routing table based on this topology.
It is important to note that the Hello protocol is bidrectional and is the means by which neighbors are discovered and acts as keepalives between neighboring routers. It also establishes and maintain neighbor relationships and elects the DR (Designated Router) and BDR (Backup Designated Router) to represent the segment on Broadcast and NBMA (nobroadcast multiaccess) networks.
Note: Hello protocols are sent periodically sent out each OSPF-enabled interface using IP multicast address 224.0.0.5. The default interval on NBMA (nonbroadcast multiaccess) networks is 30 seconds. The default interval on Broadcast, Point-to-point, and point-to-multipoint networks is 10 seconds.
What are LSAs (link-state advertisements)?
LSAs are sent out all OSPF-enabled router interfaces describing the state of the router's links. They are also packets that OSPF uses to advertise changes in the condition of a link or other OSPF routers.
Name two LSA (link-state advertisement) types?
Type 1 LSAs are router LSAs and are generated by each router for the area to which the router belongs. These LSAs describe the states of the router'links to the area (area 0 for example) and are flooded within a single area (area 0 for example).
Type 2 LSAs are network LSAs and are generated by the DR (Designated Router) and the BDR (Backup Designated Router). They describe the routers attached to a particular network and are flooded within a single area (area 0 for example).
What is the routing metric OSPF is based on?
Bandwith.
Formula: Cost = 100,000,000 / bandwidth in bits per seconds
The cost of a 100 MBbps connection would be:
1000,000,000 / 100,000,000 =1
Based on the schema above -- if adjacencies are established with only with the DR (Designated Router) and BDR (Backup Designated Router)- what is the circuit count?
Formula:
2(n - 1) where n is the number of routers in the network.
2(5 - 1) = 8 circuits.
A circuit can also be thought of as an adjacency or connection.
Count four going into the DR and 4 going into the BDR for a total of 8.
Note: OSPF avoids synchronizing between every pair of routers in the network by using a DR and BDR. This way adjacencies are formed only to the DR and BDR, and the number of LSAs sent over the network is reduced. Now only the DR and BDR have four adjacencies, and all the other routers have two.
On an OSPF-enabled router -- what is the router ID and where does an OSPF-enabled router receive its router ID?
To initialize - OSPF must be able to define a router ID. The most common and stable source for a router ID is the IP address set on the logical loopback interface that is always available. If no logical interface is defined -- then the router receives its ID from the highest IP address on the physical interfaces.
Note: If two loopback addresses are defined -- it will use the highest loopback address. Think highest logical or highest physical interface address.
Name five OSPF network types:
Broadcast networks: Ethernet/Token Ring. OSPF-enabled routers on broadcast networks elect a DR (Designated Router) and BDR (Backup Designated Router). All the routers in the network form adjacencies with the DR and BDR. Note: OSPF packets are multicast to the DR and BDR.
NBMA (nonbroadcast multiaccess) networks: Frame Relay/X.25/ATM. NBMA networks can connect more than two routers but have no broadcast functionality. These networks elect an DR and BDR. Note: OSPF packets are unicast.
Point-to-point networks: A physical DS1 (T1) for example.
Point-to-point networks connect a pair of routers and always becomes adjacent.
Point-to-multipoint networks: Point-to-multipoint networks are a special configuration of NBMA networks in which networks are treated as a collection of point-to-point links. Point-to-multipoint networks do not elect a DR or BDR. Note: OSPF packets are multicast.
Virtual links: Virtual links area special configuration that the router interprets as unnumbered point-to-point networks. The network administrator creates/defines virtual links.
What is routing?
Routing is the process in which items are forwarded from one location to another. Routing is a hop-by-hop paradigm.
A Cisco router performs routing and switching functions. Describe what each function does.
Routing is a way to learn and maintain awareness of the network topology. Each router maintains a routing table in which it looks up the destination Layer 3 address to get the packet one step closer to its destination.
The switching function is the actual movement of temporary traffic through the router, from an inbound interface to an outbound interface.
What are the three types of routes you can use in a Cisco router?
The three types of routes are static routes, dynamic routes, and default routes.
What is the difference between static and dynamic routes?
Static routes are routes that an administrator manually enters into a router. Dynamic routes are routes that a router learns automatically through a routing protocol.
What is a default route?
Also known as the gateway of last resort, a default route is a special type of static route with an all-zeros network and network mask. The default route is used to route any packets to a network that a router does not directly know about to a next-hop router. By default, if a router receives a packet to a destination network that is not in its routing table, it drops the packet. When a default route is specified, the router does not drop the packet. Instead, it forwards the packet to the IP address specified in the default route.
What is a routing protocol?
A routing protocol defines the set of rules used by a router when it communicates with neighboring routers. Routing protocols listens for packets from other participants in order to learn and maintain a routing table.
What is administrative distance?
Administrative distance (AD) is an integer from 0 to 255 that rates the trustworthiness of routing information received on a router from a neighboring router. The AD is used as the tiebreaker when a router has multiple paths from different routing protocols to the same destination. The path with the lower AD is the one given priority.
What are the three classes of routing protocols?
1) Distance vector 2) Link-state 3) Balanced hybrid
What is the AD for each of the following?
Directly connected interface 0
Static route 1
EIGRP 90
IGRP 100
OSPF 110
RIP 120
External EIGRP 170
Unknown 255
How do distance vector routing protocols function?
Also known as Bellman-Ford-Fulkerson algorithms, distance vector routing protocols pass complete routing tables to neighboring routers. Neighboring routers then combine the received routing table with their own routing table. Each router receives a routing table from its directly connected neighbor. Distance vector routing tables include information about the total cost and the logical address of the first router on the path to each network they know about.
How do distance vector routing protocols keep track of any changes to the internetwork?
Distance vector routing protocols keep track of an internetwork by periodically broadcasting updates out all active interfaces. This broadcast contains the entire routing table. This method is often called routing by rumor.
Slow convergence of distance vector routing protocols can cause inconsistent routing tables and routing loops.
What are some mechanisms that distance vector protocols implement to prevent routing loops and inconsistent routing tables?
Here are some of the ways distance vector routing protocols prevent routing loops and inconsistent routing tables:
• Maximum hop count• Split horizon
• Route poisoning• Holddowns
What is split horizon?
The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original update came.
What is convergence?
Convergence is when all routers have consistent knowledge and correct routing tables.
What is route poisoning?
With route poisoning, when a distance vector routing protocol notices that a route is no longer valid, the route is advertised with an infinite metric, signifying that the route is bad. In RIP, a metric of 16 is used to signify infinity. Route poisoning is used with holddowns.
What are hold-down timers?
Hold-down timers prevent regular update messages from reinstating a route that might have gone bad. Hold-down timers also tell routers to hold for a period of time any changes that might affect routes.
What are triggered updates?
When a router notices that a directly connected subnet has changed state, it immediately sends another routing update out its other interfaces rather than waiting for the routing update timer to expire. Triggered updates are also known as Flash updates.
What is IP RIP?
IP RIP is a true distance vector routing protocol that sends its complete routing table out all active interfaces every 30 seconds. IP RIP uses a hop count as its metric to determine the best path to a remote network. The maximum allowable hop count is 15, meaning that 16 is unreachable. There are two versions of RIP. Version 1 is classful, and version 2 is classless. IP RIP can load-balance over as many as six equal-cost paths.
What four timers does IP RIP use to regulate its performance?
Here are the four timers that IP RIP uses to regulate its performance: • Route update timer Time between router updates. The default is 30 seconds.• Route invalid timer Time that must expire before a route becomes invalid. The default is 180 seconds.• Route hold-down timer If IP RIP receives an update with a hop count higher than the metric recorded in the routing table, the router goes into holddown for 180 seconds.• Route flush timer Time from when a route becomes invalid to when it is removed from the routing table. The default is 240 seconds.
How do you stop RIP updates from propagating out an interface on a router?
Sometimes you do not want RIP updates to propagate across the WAN, wasting valuable bandwidth or giving out valuable information about your internetwork. The easiest way to stop RIP updates from propagating out an interface is to use the passive-interface global configuration command.
How do you display the contents of a Cisco IP routing table?
The show ip route command displays the Cisco routing table's contents.
What is Interior Gateway Routing Protocol (IGRP)?
IGRP is a Cisco proprietary distance vector routing protocol. IGRP has a default hop count of 100 hops, with a maximum hop count of 255. IGRP uses bandwidth and line delay as its default metric, but it can also use reliability, load, and MTU.
How do you enable IGRP on a Cisco router?
The way you enable IGRP on a Cisco router is similar to the way you enable RIP, except you specify IGRP as the protocol and add an autonomous system number. For example:
RouterA(config)#router igrp 10 (10 is the AS number)
RouterA(config-router)#network 192.168.0.0
RouterA(config-router)#network 192.168.1.0
RouterA(config-router)#network 192.168.2.0
What four timers does IGRP use to regulate its performance?
The four timers IGRP uses to regulate its performance are as follows:
• Route update timer Time between router updates The default is 90 seconds.
• Route invalid timer Time that must expire before a route becomes invalid . The default is 270 seconds.
• Route hold-down timer If a destination becomes unreachable, or if the next-hop router increases the metric recording in the routing table, the router goes into holddown for 280 seconds.
• Route flush timer[md]Time from when a route becomes invalid to when it is removed from the routing table. The default is 630 seconds.
Quick Notes - SWITCHING
What are three types of LAN traffic?
Unicasts - intended for one host.
Broadcasts - intended for everyone.
Multicasts - intended for a only a subset or group within an entire network.
What are unicast frames?
Unicast frames are the most common type of network traffic. A unicast frame is a frame intended for only one host. The only station that processes this frame is one station that has its own MAC address in the destination portion of packet.
What are broadcast frames?
Broadcast frames are frames intended for everyone. Stations view broadcast frames as public service announcements. All stations receive and process broadcast frames. In large networks, broadcasts can cause serious performance degradation in network hosts - (broadcast storm).
The destination address of broadcast frames (Data Link / Layer 2 broadcast addresses is FF-FF-FF-FF-FF-FF or alternatively all 1s in binary (11111111).
What are multicast frames?
Multicast frames address a group of devices that have a common interest. These frames allow the source to send only one copy of the frame on the network even though it is intended for several stations. Only stations that have a card that is configured by software to receive multicast frames for a particular multicast group can process a frame to that multicast address - all other stations discard multicast frames
What three major functions do Data Link Layer / Layer 2 Switches perform?
Address learning
Packet forwarding/filtering
Loop avoidance by spanning tree
What will occur when you attempt to segment a network with hubs and repeaters?
Basically, hubs and repeaters become extensions of the physical cable plant. All devices that connect to either a hub or a repeater share the same bandwidth and by definition hubs and repeaters create a single broadcast and collision domain.
Think of both devices are pass-through devices much like a electrical power-strip. Hubs and repeaters reside on the Physical Layer / Layer 1 of the OSI model where they pass 0s and 1s along the wire or up to the Data Link Layer. CSUs / Channel Service Units fall into the same category. All are regarded as unintelligent devices. No addressing takes place on the Physical layer.
What three devices are used to segment a LAN?
Router - logical addressing - IP address
Switch - physical addressing - MAC address
Bridge - physical addressing - MAC addres
What is microsegmentation?
Each workstation or network device on the network has its own dedicated segment - also known as a Collision Domain - to a switch. Each device gets the segments full bandwidth and does not have to share the dedicated segment with other devices. Collisions are reduced because each segment is its own Collision Domain.
Important: Full-duplex transmission is achieved by microsegmentation. Each device can send and receive at the same time which doubles the amount of bandwidth between nodes.
What are the three switching methods in Cisco Catalyst switches?
The three frame operating modes to handle frame switching are
Store-and-forward
Cut-through
Fragment-free
What is the Cisco Catalyst store-and-forward switching method?
In the store-and-forward switching method, the switch receives the entire frame before it forwards it. The switch reads the cyclic redundancy check (CRC) to make sure the frame is not bad. If the frame is good, the switch forwards it. Because the switch stores the frame before forwarding it, latency is introduced in the switch. Latency through the switch varies with the size of the frame.
What is the Cisco Catalyst cut-through switching method?
In cut-through switching mode, the switch only checks the frame's destination address and immediately begins forwarding the frame out the appropriate port. Because the switch checks the destination address in only the header and not the entire frame, the switch forwards a collision frame or a frame that has a bad CRC.
What is the Cisco Catalyst fragment-free switching method?
Also known as modified cut-through, fragment-free switching checks the first 64 bytes before forwarding the frame. Ethernet specifications state that collisions should be detected during the first 64 bytes of the frame. By reading the first 64 bytes of the frame, the switch can filter most collisions, although late collisions are still possible.
What is the default switching mode in Cisco Catalyst 1900 switches?
The default switching mode for the Catalyst 1900 is fragment-free.
What is half-duplex transmission mode?
Half-duplex transmission is the default mode that Ethernet functions in. In half-duplex transmission, a device can only send or receive--not do both at once. In half-duplex mode, stations are susceptible to collisions, and efficiency is rated at 50 to 60 percent.
What is full-duplex transmission mode?
In full-duplex mode, a station can send and receive at the same time. In full-duplex mode, collision detection is disabled. This mode offers 100 percent efficiency in both directions.
On a Cisco Catalyst 1900 switch, what are the default duplex settings for 10BaseT/100BaseT ports, default switching mode, and default protocols?
What are dynamic addresses on a Catalyst switch?
Dynamic addresses are addresses that the switch learns about dynamically through the learning process. If the switch does not see a MAC address for a certain amount of time, it drops the MAC address.
What are permanent MAC addresses on a Catalyst switch?
Permanent MAC addresses are entered manually by the administrator and are not aged out.
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