CIST connects all switches in the network
Common root bridge: root of CIST
Switches are grouped into regions, each region can have multiple MST Instances: VLANs are mapped to MSTI
Common Spanning Tree (CST) joins all regions together
Internal Spanning Tree (IST) is local to each region
Region root: root of IST; can specify alternate root
Regional root
The root bridge of the IST or an MSTI within an MST region is the regional root of the IST or MSTI. Based
on the topology, different spanning trees in an MST region might have different regional roots.
For example, in MST region 3 the regional root of MSTI 1 is Device B, the regional root of
MSTI 2 is Device C, and the regional root of MSTI 0 (also known as the IST) is D