Redistribution is the process of injecting routes into a routing protocol from outside the realm of the protocol. For example, if you had a router that was running EIGRP and OSPF, and you needed the routes learned by EIGRP to be advertised in OSPF, you would redistribute the EIGRP routes into OSPF. Another common example is the redistribution of static or connected routes. Because static routes are entered manually, and not learned, they must be redistributed into a routing protocol if you wish them to be advertised.
As Figure 11-1 shows, routes learned through EIGRP are not automatically advertised out of the OSPF interfaces. To accomplish this translation of sorts, you must configure redistribution within the protocol where you wish the routes to appear.
Figure 11-1. Most routing protocols do not redistribute by default
One of the main reasons protocols do not redistribute routes automatically is that different protocols have vastly different metrics. OSPF, for example, calculates the best route based on the bandwidth of the links. EIGRP, on the other hand, uses bandwidth and delay (by default) to form a very different metric. While the router could assume you wanted to redistribute, and assign a standard metric to the learned routes, a better approach is to allow you to decide whether and how routes should be redistributed.
There are two steps to redistributing ...