Two situations are possible on a link between an MST region and a non-MST region:
- either the root for all vlans is in the MST region OR
- the root for all vlans is in the non-MST region
To meet the condition, BPDU for vlans other than 1 needs to be identical or better than BPDU for vlan 1. This means e.g:
priority for vlan 1 = 8192 + system-id (1)
priority for all other vlans = 4096 + system-id (2 or higher).
This is called PVST simulation. We can disable it on the port with the command:
spanning-tree mst simulate pvst disable.
After enabling this, the port between the MST region and non-MST region will be blocked.