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.