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.
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