IP Routing

How RIP Finds Shortest Paths

All DV protocols essentially operate the same way: routers exchange routing updates with neighboring (directly connected) routers; the routing updates contain a list of network numbers along with the distance (metric, in routing terminology) to the networks. Each router chooses the shortest path to a destination network by comparing the distance (or metric) information it receives from its various neighbors. Let’s look at this in more detail in the context of RIP.

Let’s imagine that the network is cold-started -- i.e., all three routers are powered up at the same time. The first thing that happens after IOS has finished loading is that the router checks for its connected interfaces and determines which ones are up. Next, these directly connected networks are installed in each router’s routing table. So, right after IOS has been loaded and before any routing updates have been exchanged, the routing table would look like this:

NewYork>sh ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default Gateway of last resort is not set C 171.16.1.0 is directly connected, Ethernet0 C 171.16.250.0 is directly connected, Serial0 C 171.16.251.0 is directly connected, Serial1 Chicago>sh ...