STP - MST and PVST+ selecting root port
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.
When an MST switch receives PVST+ BPDUs, 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 on all configured VLANs except those from VLAN 1, must be the same or superior compared to the PVST VLAN 1 BPDU.