CCNA study notes on routing protocols

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40. Default administrative distances some important routing protocols are as below:

Route Source

Default Distance

Directly connect Interface


Static Route








An administrative distance of 0 represents highest trustworthiness of the route.

An administrative distance of 255 represents the lowest trustworthiness of the route.

Routed and Routing protocols:

42. There are three ways a router learns how to forward a packet:

43. To enable the Cisco IOS to forward packets destined for obscure subnets of directly connected networks onto the best route, you use "ip classless" command.

44. There are broadly three types of routing protocols:

45. Distance vector protocol depends only on Hop count to determine the nearest next hop for forwarding a packet. One obvious disadvantage is that, if you have a destination connected through two hops via T1 lines, and if the same destination is also connected through a single hop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!

46. RIP (and IGRP) always summarizes routing information by major network numbers. This is called classfull routing

47. Convergence is the term used to describe the state at which all the internetworking devices, running any specific routing protocol, are having identical information about the internetwork in their routing tables. The time it takes to arrive at identical information of the internetwork is called Convergence Time.

48. RIP,RIP2, and IGRP use distance vector algorithms.

RIP2 transmits the subnet mask with each route. This feature allows VLSM (Variable Length Subnet Masks) by passing the mask along with each route so that the subnet is exactly defined.

49.Routing metrics used by IGRP:

Bandwidth, MTU, Reliability, Delay, and Load.

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

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

52. 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.

53. 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.

54. 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.

55. 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.

56. Cisco router boot configuration commands:

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