Spanning-tree MSTR port role

The way it works when connecting MST and PVST domain is that the MST switch at the edge of the domain will simulate PVST+ by sending same BPDU for all VLANs configured on the trunk towards the PVST+ switch.

The CIST replicates BPDU for every VLAN. Because there is not one instance per VLAN for MST the sys ID is set to 0 on these BPDUs. When connecting MST and PVST+ domain VLAN 1 is used to connect the domains.

When the PVST+ side sends a BPDU with priority 0 for VLAN 1 it is actually 1 because of extended system ID. If VLAN 2 is also configured then PVST+ side sends BPDU with ID of 2 because of the extended system ID.

The way PVST simulation works on MST switch is that the port can only have one role because there is not one instance running per VLAN. The CIST is now sending BPDU that it knows a root which has a priority of 1. This means that MST switch should be root forwarding but then it sees a BPDU with a priority of 2. Because the MST switch has better root information it should become designated forwarding on the port but the port can only have one role so the PVST simulation fails.

To make this work the PVST+ side must have worse priority on VLAN 1 than all the other VLANs on the trunk. The following configuration will work:

spanning-tree vlan 1 priority 4096
spanning-tree vlan 2-4094 priority 0

This way the PVST simulation will work and the following log message should be seen:

%SPANTREE-2-PVSTSIM_OK: PVST Simulation inconsistency cleared

More information can be found in the following link.

If anything it may just need to reconverge. About 45 seconds of interruption IIRC. If you can, while you are changing it on all the switches anyway, use rapid PVST. That will allow for faster convergence.