Redistribution

Route redistribution might be required in an internetwork because multiple routing protocols must coexist in the first place. Multiple routing protocols might be a necessity because of an interim period during conversion from one to another, application-specific protocol requirements, political reasons, or a lack of multivendor interoperability.

A major issue with redistribution is the seed metric to be used when the routes enter the new routing protocol. Normally, the seed metric is generated from the originating interface. For example, EIGRP would use the bandwidth and delay of the originating interface to seed the metric. With redistributed routes, however, these routes are not connected to the router. Some routing protocols feature a default seed metric for redistribution, whereas others do not. Here is a list of the defaults for the various protocols. Note that Infinity indicates a seed metric must be configured; otherwise, the route will not be used by the receiving protocol.

Protocol ------ Default Seed Metric
OSPF ------ 20; except BGP, which is 1
IS-IS ------ 0
RIP ------ Infinity
IGRP/EIGRP ------ Infinity