MST and PVST+ Interoperability

Hello Joyce

The particular scenario you are describing is one where SW1, using PVST, sends BPDUs to SW2 and SW3 using PVST BPDUs, one for each VLAN. These BPDUs indicate that SW1 has a priority of 4096 (as shown in the related section in the lesson). What happens in this case?

MST will perform a check that must be passed in order for these BPDUs to be accepted. THe check states that "in order to become a root port, all PVST BPDUs except those from VLAN 1, must be the same or superior compared to the PVST VLAN 1 BPDU. HOwever, here, this is not the case since VLANs 10 to 60 have a priority of 32768, while the PVST BPDUs are sending a superior priority.

So SW2 and SW3 will use the VLAN 1 BPDUs received from SW1 to determine their port states (root or not) only if this check is fulfilled. If it is not, there is an error and the ports are put into blocking state.

Other than the video, this is more thoroughly explained in the PVST+ Root Bridge section of the lesson.

I hope this has been helpful!