STP - what bridge ID does the MST region use
There are various types of Spanning Tree Protocol (STP). Multiple spanning tree or MST and Per VLAN Spanning Tree or PVST+ are two types that are often used. These two protocols are different, but they are compatible.
On links that connect the MST region with the PVST+ region, the PVST+ region views the whole MST region as a single large switch. When it does so, it advertises the bridge ID of the root bridge in the IST as the bridge ID of the whole MST region.
See also: STP - MST Virtual Bridge and its role in interfacing with other STP topologies.
Links:
https://forum.networklessons.com/t/multiple-spanning-tree-mst/1101/72?u=lagapides
https://networklessons.com/spanning-tree/mst-pvst-interoperability