Route redistribution is one of the more powerful, yet complex, of a router's capabilities. In essence, route redistribution is a fairly simple concept. The gateway routers participate in route calculation and convergence for the networks of which they are members. Routing information for that network is summarized and shipped to a neighboring network (albeit one that runs a different routing protocol). In practice, configuring a route redistribution scheme is fairly complicated.
One of the main reasons for the complexity of route redistribution is the fundamental differences that can exist between the various routing protocols. As demonstrated throughout Part III, "'Routing Protocols," such protocols can make their routing decisions using very different algorithms (distance-vector versus link-state) as well as a surprising variety of actual routing metrics.
Routing information can only be distributed if it is expressed in a form that is understood by the receiving network's routing protocol. Attempting to ship IGRP composite metrics to a RIP network would not be very productive. RIP uses only a single metric: hop count. A RIP network wouldn't understand the IGRP metrics or any other protocol's metrics. Routing information that is to be redistributed to a RIP network must, therefore, be expressed in terms of hop count. Therefore, route redistribution requires the use of a metric common to both networks' routing protocols. In other words, redistributing routing information is a least- common-denominator exercise.
You must specify a default metric that will be used to express a route's cost. Routes redistributed to external networks use this default metric to calculate routes and make routing decisions. It is imperative, therefore, that you understand the routing protocols that you will be redistributing information to (and from) before you select a default metric. If you understand the routing protocols and their respective metrics, you shouldn't have any problem with configuring a default metric.
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