CIST Root Election

[yassineelaissaoui955]

How the CIST root election works?

[MHM Cisco World · Accepted solution]

https://youtu.be/h8iuNNfiDW4?si=IKd9u6xrfGYad-v3 <<- check this video' if it not clear let me know 

MHM

[M02@rt37 · Accepted solution]

Hello @yassineelaissaoui955 

The CIST root election follows the same principles as standard STP/RSTP but applies across MST regions.

Each switch has a BID composed of a Bridge Priority, an Extended System id, and a MAC @. The switch with the lowest BID (smallest priority, and if tied, lowest MAC address) is elected as the CIST Root Bridge. All switches exchange BPDUs , and if a switch receives a BPDU with a lower BID, it updates its root bridge information and forwards the BPDU.

The election finalizes when all switches agree on the CIST Root Bridge, which has a root Path Cost of 0, while other switches calculate the cost to reach it. The CIST ensures spanning tree consistency across MST regions and external (RSTP/STP) bridge, forming a single logical spanning tree for the entire network.