Within an MSTP region,
the bridge with the lowest path cost to the CIST root bridge is the CIST regional root
bridge If the CIST root bridge is inside an MSTP region, that same bridge is the CIST
regional root for that region because it has the lowest path cost to the CIST root. If the
CIST root bridge is outside an MSTP region, all regions connect to the CIST root through
their respective CIST regional roots.
