MST - Root port selection for switches outside MST topology

When an MST topology connects to a switch that is outside of the MST domain, it will select its root port based on the BPDUs received from multiple switches within the MST domain. However, the BPDUs that are sent are those that correspond to the Internal Spanning Tree (IST) of the MST region. By default, the IST uses instance 0 to run the IST. The IST runs RSTP. Thus, the MST region, as seen from outside, uses the root bridge of the IST for interactions with such switches.

See also: STP - MST Virtual Bridge and its role in interfacing with other STP topologies.

https://networklessons.com/cisco/ccie-routing-switching-written/multiple-spanning-tree-mst

https://forum.networklessons.com/t/multiple-spanning-tree-mst/1101/81?u=lagapidis