STP Convergence/Topology Changes

[mmelbourne]

We are getting some reports of network 'blips'. The only correlation is a STP topology change in the VLAN on the distribution switches. "show spanning-tree vlan <X> detail" shows the port this came from, but following this port to its attached access layer switch suggests the notification came from the distribution switch (back across the same link). Has anyone seen anything similar? Is there any other way to try and track this down?

[Ian Jay]

Its most likely the downstream switch is originating the TCN and then a corresponding configuration BPDU is received. Unfortunately there's nothing on the originating switch to tell you it has generated the TCN. This is where detective work comes into play.

Logging messages of interest:

  - network port flapping

  - root bridge changing

  - native vlan mismatch

  - mac address flapping

Spanning tree output:

  - interfaces in "Shr" status

  - interfaces in Bound status

Interface output:

  - CRC or input errors

  - half duplex status

  - output drops

Similar symptoms can also result from switches sending latent BPDUs. This can cause momentary root bridge changes as BPDU updates are not received in time. Depending on the number of vlans you can enable BPDU debugging to narrow a possible timing issue down