EIGRP updates for equal metric routes

Unanswered Question
Feb 25th, 2010

Hey all,

This is kind of a split-horizon/poison-reverse question, but here we go.

In a very simple network, lets say we have the following:

  A

/   \

B   C

Let's say A learns the default route 0.0.0.0/0 from both B and C, and they both have the same exact metric, and those are the same links (both 1 gig for example).

Will A advertise to C the route it learned from B, and will A advertise to B what it learned from C if both routes in A's routing table are equal cost?

I spent some time in my lab, and when A was a 6500 with equal cost metrics, there was no updates from A to B / C. B/C did NOT have an entry in the topology table for 0.0.0.0/0 via A.

However,

When I made my 4507 A, I DID see it update only ONE of the neigbhors.

Whichever came up first it did not update. So if B sends A the route, and then C sends A the route, A sent C a route about B, but it didn't send B anything. C did have a topology table entry for 0.0.0.0/0 via A (it was NOT a FS, however -- understandably so).

Now I don't know why I'm seeing different behavior, so that's why I'm here. I think if it's equal metric the switch should NOT send an update back.

Thanks for any / all feedback!

I have this problem too.
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Giuseppe Larosa Mon, 03/01/2010 - 11:30

Hello Tdistlists,

interesting note about different EIGRP behaviour in two different Cisco multilayer switches.

It would be helpful if you could provide some more details like supervisor model and IOS image running on the two boxes.

Hope to help

Giuseppe

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