Within an MSTP region, the bridge with the lowest path cost to the CIST root bridge is the CIST regional root bridge.
The path cost, also known as the CIST external path cost, is a function of the link speed and number of hops. If there is more than one bridge with the same path cost, the bridge with the lowest bridge ID becomes the CIST regional root. If the CIST root is inside an MSTP region, the 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 is outside an MSTP region, all regions connect to the CIST root via their CIST regional roots.
The total path cost to the CIST root bridge from any bridge in an MSTP region consists of the CIST internal path cost (the path cost of the bridge to the CIST regional root bridge) and the CIST external path cost. To build a loop-free topology within a region, the CIST uses the external and internal path costs, and the MSTI uses only the internal path cost.
Looking at MSTP region 1 in Closeup of MSTP Region 1, the total path cost for the bridge with ID 60 consists of an external path cost of A and an internal path cost of E.