STP - MST viewed as a single switch by PVST+
When deploying Spanning Tree Protocol (STP) in an environment where both Multiple Spanning Tree (MST) and Per VLAN Spanning Tree plus (PVST+) are operating, it is important to understand how the root port is selected at the border between the two STP protocols. This is achieved by exchanging BPDUs between MST and PVST+ switches.
In this case, the MST topology is viewed by the PVST+ switches as one big switch. Thus, from the point of view of PVST+, there can only be a single root port from the MST topology facing the PVST+ topology. This means that an MST topology with multiple switches that has multiple connections to a PVST+ topology must necessarily have only one root port facing the PVST+ topology.