From cwhite's wiki
- CIST Root is the bridge that has the lowest Bridge ID among ALL regions. This could be a bridge inside a region or a boundary switch in a region.
- CIST Regional Root is a boundary switch elected for every region based on the shortest external path cost to reach the CIST Root. Path cost is calculated based on costs of the links connecting the regions, excluding the internal regional paths. CIST Regional Root becomes the root of the IST for the given region as well.