STP - Understanding MST regional root identifier and proposal-agreement process

In MST (Multiple Spanning Tree) protocol, the "Regional Root Identifier" is used to identify the root bridge for each instance within an MST region. This identifier combines the bridge priority and the MAC address (System ID Extension + MAC). The local switch's Bridge Identifier is found in the "CIST Bridge Identifier" field, which is part of the Common and Internal Spanning Tree (CIST) that operates within an MST region and connects with other MST regions and spanning tree protocols.

During the MST negotiation process, the Proposal and Agreement flags play a crucial role. When a switch sends a BPDU with the Proposal flag set, it proposes to the downstream switch that it should be the root port. If the downstream switch agrees, it responds with BPDUs that have the Agreement flag set, facilitating a rapid agreement on the state of the ports.

https://forum.networklessons.com/t/multiple-spanning-tree-mst/1101/90?u=lagapidis

https://networklessons.com/spanning-tree/multiple-spanning-tree-mst/