Monday, March 9, 2009

Redistribution Basics

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

Best Regards,
Deepak Arora

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