Fundamentals of Route Redistribution

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Route redistribution involves placing the routes learned from one routing domain, such as RIP, into another routing domain, such as EIGRP. When this occurs, you have several issues to address, one of which is metrics.

Each routing protocol has its own way of determining the best path to a network. RIP uses hops, and EIGRP and IGRP both use a composite metric of bandwidth, delay, reliability, load, and MTU size. Because of the differences in metric calculations, when redistributing routes, you lose all metrics and must manually specify the cost metric for each routing domain. This is because RIP has no way of translating bandwidth, delay, reliability, load, and MTU size into hops, and vice versa. Another issue to address with route redistribution is that some routing protocols are classful, meaning that the routing protocol does not send subnet mask information in the routing updates (for example, in RIP and IGRP). In addition, some protocols are classless, meaning that the routing protocol does send subnet mask information in the routing updates (for example, in EIGRP). This poses a problem when variable-length subnet masking (VLSM, in which you use a netmask other than the default netmask for the IP address) and classless interdomain routing (CIDR, sometimes referred to as "supernetting" or route summarization) routes need to be redistributed from a classless routing protocol into a classful routing protocol.

Route redistribution can get very complex and can introduce many problems. Sometimes the solutions to the problems are even more complex than the problems themselves . This chapter keeps the situation as basic as possible, to help you understand the issues involved in route redistribution without being overwhelming. Just keep the issues outlined previously in mind as you configure route redistribution in the lab that follows . The lab discusses solutions to the problems as they arise.

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