Electing a RB (Root Bridge/Switch)

Criteria: Switch wit the Least Priority

Tie Breaker: MAC Address

• As mentioned earlier that to prevent loop STP puts an interface either in Forwarding or Blocking mode. To do that it elects a RB first.

• Initially when all the Switches come up they start exchanging BPDU (Bridge Priority + MAC Address) & all the switches will put their own BID as a Root Bridge ID.

• If a Switch hears a Hello that lists a lower BID, then that Switch stops advertising it's own BID as RB ID & starts forwarding their own BID as well as the Superior Bridge's Hellos as a Root Bridge ID.

• Eventually by exchanging Hellos, Switch with the least Priority (32768 default Priority) becomes a RB. If Priority is same then the election goes on to the MAC Address which acts as a tie breaker.

• By default all the ports of a Root Bridge will be marked as Designated Ports & hence will always be in the forwarding state

Reference:

• CCNA 200-301 OCG, Volume 1, Pg. 218,219- Wendell Odom.

• RST Notes, Volume 2