While in Fabric Cluster mode, changing the spanning-tree interface configuration without first shutting down the interface can cause spanning-tree to go discarding
Software Release: 5.0.1c Fixed in Version: 5.0.1d, 5.0.2, 6.0.1
The spanning-tree state was incorrect in VCS 999 because of the misconfiguration between Core1 and Core2. After the misconfiguration was corrected, spanning-tree did not converge correctly since it was already in a precarious state.
The VDX should have been able to recover spanning-tree state properly after fixing this misconfiguration listed above. You should not have to bounce the interface to correct the state.
The best practice if making changes to spanning-tree interface configuration you should first shutdown the interface before making the configuration change.
To avoid possible misconfigurations between VCS members port-channel interface configuration, it isbest to use Logical Chassis mode as these types of misconfigurations are not possible between VCS members.
To prevent this issue in the future, shutdown the port-channel interface first, make the configuration change, then no shutdown the interface.
If you have found yourself already in this situation after reconfiguring the interface spanning-tree configuration, shut/no shut the port-channel interface on Core1 to reset the spanning-tree state.