loopback problem..

Answered Question
Aug 14th, 2007


I have a really strange loopback problem..

Sometimes, when my network topology change (like a link goes down), about half of my 802.1q trunks over the network goes err-disabled because of a loopback detection.

I've triple-checked... There's absolutly no loopback not managed by STP on my network.

I really don't understand why only half of my switches are detecting the problems and others don't...

I have about 120 switches on the network (80X2950 (12.1(20)), 30X2960 (12.2(25)), 10X3750 (12.2(20)))

To patch the problem, I was considering stopping the errdisable for the loopback cause or at least, making a low recovery interval for this cause...

Someone have any advices to give me??


Emmanuel Bouchard

I have this problem too.
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May require more info to identify what is the problem but for a start -

Hope you have not enabled spanning-tree portfast on any port where you are connecting another switch.

Which link going down is affecting the network? Is it the same link which goes down always which creates this situation?

From the logs can you find out which ports go down first?

any difference in the trunk configs of switch which goes down and and the switch which does not go down.


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