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TCN puzzling

Unanswered Question
Mar 10th, 2006
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I've got portfast configured on all end-user ports on several 2950s, which are redundantly hosted by two 6513s. I am using mst and have nearly 200 vlans configured. I have around 90 2950s. I am running two instances of mst and have the vlans spread equally over them both. I keep get topology change notifications at the cores from two sources: a particular 2950 (don't know why) and a L2 port-channel. Any ideas how on what info I need to look at to figure out what is causing these TCNs.

Is it possible that if vlans from both mst instances are running on a switch, then only vlan(s) from one instance are running would cause a TCN? Thanks

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leonvd79 Sun, 03/12/2006 - 23:30
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I am not aware of your topology and STP design.

It is best you start by determining the TCN initiator.

Issue "show spanning-t vlan detail" on the root of the spanning-tree. And see when and from where the last TCN was received.

From there on go to the switch that sent the TCN to the root and issue the same command. Look at the forward transitions on p2p ports and bpdu counters in particular, if you find high amount of transitions or sent and received BPDU's try to debug the STP events on that switch.

debug spanning-tree mstp

debug spanning-tree bpdu

debug spanning-tree events

debug spanning-tree synchronization

Observe the output in the debug logging and define the source of the problem.



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