STP - MST CIST Root
Within the context of the Spanning Tree Protocol (STP), when two MST regions are interconnected, the resulting CIST root will be chosen based on the following criteria:
If the CIST root is outside the region, (i.e. not within the local MST region) the protocol selects one of the MST switches at the boundary of the region as the CIST regional root.
…within each MST region, the IST root should be the switch with the lowest path cost to the CST root. This ensures that traffic within the region takes the most efficient path to the CST root, minimizing the overall network path cost.
Links
https://forum.networklessons.com/t/multiple-spanning-tree-mst/1101/98?u=lagapidis
https://forum.networklessons.com/t/multiple-spanning-tree-mst/1101/97?u=lagapidis