Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Topology change in one VLAN of an MSTP instance affects all VLANs in that instance?

I have a question regarding MSTP.


Say I have 3 switches, A B and C.  A and B are connected to each other using a trunk link and A is the STP root.  Then, I connect switch C to switch B using an access port which is on vlan 100.  And, switch C's priority is lower than both A and B for VLAN 100.


I believe that if all these switches are running R-PVST, the behavior is that C will cause B to perform an RSTP sync-proposal-agreement but only for vlan 100.  This will cause a momentary outage for ONLY vlan 100 between A and B.  All other VLANs will not be affected.


Let me know if I got it right for MSTP.  In MSTP, since all VLANs are in a single instance, a change to one instance affects all the VLANs of that instance.  There will be a momentary outage for all VLANs between A and B.  Is that right?


Thank you in advance!