Symptom

On recovery from a dual-active scenario (i.e. when keep-alive and peer-link comes back up), STP puts some of the vPC port-channels into the blocking state.

Conditions

The issue occurs when peer-link as well as peer keep-alive goes down, but both the vPC peer switches and port-channels are still up and running. Due to the dual failure, both the vpc peer switches assume that the other side is dead. They both take over the role of Master and keep their vpc port-channels UP. On recovery from such a scenario (i.e. when keep-alive and peer-link comes back up), STP goes into inconsistent state puts some of the vPC port-channels into the blocking state. The issue happens only when the vpc-pair switch is not the stp root.

Workaround

Either of the following workarounds can be used to avoid this issue: 1. Keep STP root on one of the vpc switches. 2. Avoid dual-active scenario. Try to put keep-alive mechanism in such a way that it doesn't fail, when the peer-link goes down but the vpc port channels are still up.

Further Problem Description