Allied Telesis AT-GS950/48 - Manual (Page 386)

Appendix A: MSTP Overview
386
A port connected to a bridge running STP or RSTP is considered to be a
boundary port of the MSTP region and the bridge as belonging to a different
region.
An MSTP region can be considered as a virtual bridge. The implication is that
other MSTP regions, and STP and RSTP single-instance spanning trees,
cannot discern the topology or constitution of an MSTP region. The only
bridge they are aware of is the regional root of the CIST instance.