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?