IGRP Redistribution

IGRP Redistribution
At this point, we will only briefly cover redistribution for IGRP. Redistribution will be covered in
greater detail in Chapter 10. You may be wondering what redistribution is. Redistribution is the
process in which routes learned by one routing method or protocol—whether static or dynamic—
are shared with a dynamic routing protocol. For routing protocols that use AS numbers to distinguish
routing instances on the same router, these methods could actually be the exact same
dynamic routing protocol, with redistribution occurring between the different AS numbers. IGRP
and EIGRP certainly qualify as examples of these routing protocols. Let’s look at Figure 4.1.
As you can see in this example, we have Router2, which has IGRP 100 and EIGRP 150 running
on it. Router2 knows about all the routes in both IGRP 100 and EIGRP 150. Router1 knows only
about the routes in IGRP 100, and Router3 knows only about routes in EIGRP 150.
So what can we do in order for Router1 and Router3 to know about the same routes? We can
redistribute IGRP 100 into EIGRP 150 on Router2. This will give Router3 the same routes as
Router2. If we stopped here, this would be known as one-way redistribution. One-way redistribution
means that redistribution occurred in only one direction between the two routing protocols.
If we want all routers in this network to have the same routes, we must set up what is known
as mutual redistribution. Mutual redistribution is when redistribution occurs in both directions
between the routing protocols involved. In this case, we would need to redistribute IGRP 100
into EIGRP 150 and EIGRP 150 into IGRP 100 on Router2. Once converged, all the routers on
this network will know the same routes.