Allied Telesis AT-9000 Series - Page 726

Chapter 47: Multiple Spanning Tree Protocol (MSTP)
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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.