Giuseppe Larosa Sat, 07/05/2008 - 11:36
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Hello Raja,

you can face suboptimal paths in redistibution scenarios with multiple routers that take part in both routing protocols.

For some IP Prefix on some node the usual parameters used to choice the best path can lead to a choice that is not the best one.

A router first looks at the prefix len taking the longest (most specific) prefix then it looks at the administrative distance( lowest wins ) , if both matches it is comparing two routes from the same routing protocol (with default configurations))and now the protocol metric is compared and the lowest wins.

For example Router R4 is connected to a RIP routing domain via interface fas0/0 and to a OSPF domain via fas0/1.

Suppose the R4 is not doing any redistribution between the two protocols but then somewhere another router R2 is doing redistribution = advertising the RIP routes into OSPF.

Suppose that is a network attached to R4's RIP neighbor on fas0/0.

As soon as R4 learns a network via OSPF via interface fas0/1 it will install it in its routing table and will send packets for net out fas0/1 taking the longer path instead of using fas0/0.

This is an example of suboptimal path.

RIP admin distance = 120, OSPF admin distance 110

hope to help


Richard Burts Sat, 07/05/2008 - 12:40
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While the explanation by Giuseppe has many good points and concepts, I will take a different and more simple approach to the explanation.

First let us be clear about the term "optimal path" which is the best path toward a destination.

Then keeping in mind that "sub" generally means lower or less than, it becomes clear that "suboptimal path" is a path toward a destination that is less good than the optimal path.




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