mstp topology change

[Alexander Proskurnin]

Hi!

I have a question about MAC invalidation during topology change. If I have one region with several instances in it and switch notices topology change for only one instance, will it invalidate MACs for all instances within the region or only MACs for particular instance will be affected?

Thanks!

[Peter Paluch · Accepted solution]

Hi Alexander,

Quoting from the IEEE 802.1Q-2005 standard, Section 13.17:

Changes in the active topology of any MSTI do not change end station locations for the CIST or any other MSTI, unless the underlying changes in the physical topology that gave rise to the reconfiguration also cause those trees to reconfigure. Changes to the CST, i.e., the connectivity provided by the CIST between MST Regions, can cause end station location changes for all trees. Changes to an IST can cause CST end station location changes but do not affect MSTIs in that Region unless those trees also reconfigure.

In other words, if the topology change is concerned with a single MSTI, only the entries related to VLANs sharing that particular instance will be flushed. If the CST experiences a topology change (i.e. the boundary ports change states or receive TCNs themselves), this change may optionally affect all instances within the MSTP region. (Not trying to provide an exhaustive explanation here, though.)

Best regards,

Peter