IP Routing

Redistribute Routes

Ideally, you should run only a single IGP in any given network. However, as networks evolve they often end up running multiple routing protocols. How does this happen? After all, shouldn’t the routing engineer select one routing protocol and stick with it as the network grows?

Consider a network running RIP. The network is to be extended to support a new business area, and the routing engineers decide not to use RIP for the extension because of its long convergence times. Instead, they deploy EIGRP on the extension, while continuing to use RIP on the legacy network.

In another scenario, consider two corporations that merge and ask their network engineers to join their networks. One network may have been running OSPF and the other IGRP. The two routing domains in this scenario are described in Figure 8-2.

Figure 8-2. A network with two routing domains

For the network in Figure 8-2 to remain cohesive -- i.e., for end-stations in one domain to reach end-stations in the other domain -- router R must perform some kind of “translation,” taking the routes from the OSPF domain and conveying them into IGRP, and vice versa.

This “translation” of routing information from one domain (or routing protocol) to another is known as route redistribution . Note that route redistribution is a one-way translation of routing information from one routing protocol to another. The two-way ...