IP Routing

Chapter 3. Interior Gateway Routing Protocol (IGRP)

The second Distance Vector protocol that we will examine is the Interior Gateway Routing Protocol, or IGRP. IGRP and RIP are close cousins: both are based on the Bellman-Ford Distance Vector (DV) algorithms. DV algorithms propagate routing information from neighbor to neighbor; if a router receives the same route from multiple neighbors, it chooses the route with the lowest metric. All DV protocols need robust strategies to cope with bad routing information. Bad routes can linger in a network when information about the loss of a route does not reach some router (for instance, because of the loss of a route update packet), which then inserts the bad route back into the network. IGRP uses the same convergence strategies as RIP: triggered updates, route hold-downs, split horizon, and poison reverse.

IGRP has been widely deployed in small to mid-sized networks because it can be configured with the same ease as RIP, but its metric represents bandwidth and delay, in addition to hop count. The ability to discriminate between paths based on bandwidth and delay is a major improvement over RIP.

IGRP is a Cisco proprietary protocol; other router vendors do not support IGRP. Keep this in mind if you are planning a multivendor router environment.

The following section gets us started with configuring IGRP.

Getting IGRP Running

TraderMary’s network, shown in Figure 3-1, can be configured to run IGRP as follows.

Figure 3-1. TraderMary’s network