What happens if MST's own BPDU is not superior compared to PVST+ BPDUs?

In the context of Multiple Spanning Tree (MST) and Per VLAN Spanning Tree (PVST+), if an MST instance receives a BPDU that is not superior to its own, the specified behavior is to block the boundary interface.

MST operates by sending its own BPDUs and expecting superior BPDUs from other MST instances for proper functioning. When MST receives a PVST+ BPDU that is deemed less favorable, it signifies that the PVST+ instance has selected a path that is not preferred within the MST region. As a result, to prevent potential loops and ensure stable network topology, the MST boundary interface is placed into a blocking state. This behavior maintains the integrity of the MST's control over the traffic flow while adhering to the network's overall spanning tree protocol configuration.

Blocking the boundary interface ensures that there is no unintended forwarding of traffic through the MST region when competing with PVST+, thus allowing the network to remain efficient and loop-free.