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Cisco® CCNA Exam Cram Notes : Eigrp Routing Metrics, Routing Loops Prevention, And Router Boot Config Commands.

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51.Routing metrics used by IGRP:

Bandwidth, MTU, Reliability, Delay, and Load.

  • Bandwidth: This is the smallest bandwidth between the source and destination.
  • Delay: This is the cumulative interface delay along the path.
  • Reliability: This is the worst case reliability between source and destination based on keep alives.
  • Loading: This is the worst case load on a link between source and destination based on bps.
  • MTU (Maximum Transmission Unit): This is the maximum message length that is acceptable to all links on the path. The larger MTU means faster transmission of packets.

52. The metric limit for link-state protocols is 65,533

53. Following are the possible solutions for preventing routing loops.

  • Split Horizon - based on the principle that it is not useful to send the information about a route back in the direction from which the information originally came.
  • Poison Reverse - A router that discovers an inaccessible route sets a table entry consistent state (infinite metric) while the network converges.
  • Hold-down Timers - Hold down timers prevent regular update messages from reinstating a route that has gone bad. Here, if a route fails, the router waits a certain amount of time before accepting any other routing information about that route.
  • Triggered Updates - Normally, new routing tables are sent to neighboring routers at regular intervals (IP RIP every 30 sec / and IPX RIP every 60 sec). A triggered update is an update sent immediately in response to some change in the routing table. Triggered updates along with Hold-down timers can be used effectively to counter routing loops.

54. IP RIP based networks send the complete routing table during update. The default update interval is 30 seconds. IGRP update packet is sent every 90 seconds by default.

55. For IGRP routing, you need to provide the AS (Autonomous System) number in the command. Routers need AS number to exchange routing information. Routers belonging to same AS exchange routing information. OSPF, and IGRP use AS numbers.

56. CDP stands for Cisco Discovery Protocol. This protocol is proprietary of Cisco. CDP runs SNAP (Sub network Access Protocol) at the Data Link Layer. Two Cisco devices running two different Network layer protocol can still communicate and learn about each other.

57. Show IP protocol: This command will show information on RIP timers including routing update timer (30sec default), hold-down timer (default 180sec). It also displays the number of seconds due for next update (this is fraction of update timer). This command also gives the network number for which IP RIP is enabled, Gateway, and the default metric.

Show IP route: This command will display the IP routing table entries. In addition, it displays the Gateway of last resort (if one is assigned). It also displays the codes used for various types of routes. Some of the important codes are:

C: directly connected;

S: Statically connected

I : IGRP

R : RIP

2. Show IP interface: This command shows you interface-wise information such as IP address assigned to each interface, whether the interface is up, MTU etc.

3. Debug IP RIP: Debug IP RIP will turn the RIP debugging ON. This will display a continuous list of routing updates as they are sent and received. This leads to lot of overhead, which is the reason that you use "undebug ip rip" to turn-off debugging as soon as you finish with debugging.

58. Cisco router boot configuration commands:

  • boot system - This is a global command that allows you to specify the source of the IOS software image to load. If you configure more than one source, attempts are made to load the IOS from the first command in the configuration to the last successively. If the first fails, the second boot command is used.
  • boot system rom - Loads IOS from ROM.
  • boot system flash - Loads the first file from flash memory.
  • boot system tftp <file name> < tftp_address >-Loads IOS with a filename <file name> from a TFTP server.

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