7.5 Using Multiple Routing Protocols
7.5.1 What is redistribution
Thus far, this chapter has looked at networks that use a single routing protocol. There are times, however, when you will need to use multiple routing protocols. The following are a few reasons why you might need multiple protocols:
  • When you are migrating from an older Interior Gateway Protocol (IGP) to a new IGP, multiple redistribution boundaries may exist until the new protocol has displaced the old protocol completely. Dual existence of protocols is effectively the same as a long-term coexistence design.
  • You want to use another protocol but need to keep the old protocol because of the needs of host systems.
  • Different departments might not want to upgrade their routers or they might not implement a sufficiently strict filtering policy, so you can protect yourself by terminating the interior route protocol.
  • If you have a mixed router vendor environment, you can use a Cisco protocol in the Cisco portion of the network and then use a common protocol to communicate with the devices that are not Cisco devices.

When any of the aforementioned situations arise, Cisco routers allow internetworks using different routing protocols (referred to as ASs) to exchange routing information through a feature called route redistribution. Redistribution is defined as the capability for boundary routers connecting different ASs to exchange and advertise routing information received from one AS to the other AS.

Within each AS, the internal routers (in the case of the main figure. the internal IGRP and EIGRP routers) have complete knowledge about all subnets that make up each network. The router interconnecting both ASs is called a border router, and it has both IGRP and EIGRP processes active. The border router is responsible for advertising routes learned from one AS into the other AS.

In the main figure, network 192.168.5.0 is known via the S0 interface. The routing table for the router in AS 300 contains routes, such as 192.168.5.0 and 172.16.0.0, that are summarized at network boundaries. These routes are indicated by the "D" for EIGRP and "EX" for an external route that was learned from redistribution.