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Nortel 5520 STG for most ports changes to 1

Nortel 5520 STG for most ports changes to 1

(OP)
I have a stacked 5520 that is causing problems. About 4 switches on the network (Cisco 4507 core, variety of Nortel/Cisco edge) drop off for ~30sec randomly. This has occurred before and is to do with STG. I've narrowed it down to the stacked 5520 being the cause and when it goes offline briefly it brings down 3 others.

I've checked the STG settings and found some ports were not enabled so I enabled them and set all to a PathCost of 10, except the trunk which is set to 1. But after these 'blips' the PathCost of nearly every port changes to 1. Any ideas why? How can I stop this from happening and taking the other switches down?

RE: Nortel 5520 STG for most ports changes to 1

(OP)
Ignore that! I was looking at one of the other affected switches. The PathCost is not being altered when the issue occurs.

But the issue still exists where one switch loses connectivity briefly and takes 3 others with it. Any ideas why?

RE: Nortel 5520 STG for most ports changes to 1

Check each type are using a version of STP that inter-operates between Cisco/Nortel.

Check your network topology, make sure it's a star, and eliminate loops - replace with aggregated links where possible.

RE: Nortel 5520 STG for most ports changes to 1

Is your root bridge moving around during the blips? I'm wondering if this is a root guard or loop guard feature in Cisco that the 5520s aren't playing well with.

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