Choosing a Root Port

There are 3 different port states on a switch which are designated port (the forwarding port), alternative port (backup/alternative port that is blocked to prevent switching loop) and root port. There is only 1 root port in a switch, possible multiple blocked and designated ports (except the root bridge which has all the ports on designated).

A root port is the port needed to connect a particular switch to its root (either directly or indirectly). There is a method in choosing a root port in a switch:

1. Choose the port that receives a superior BPDU (a BPDU from a switch with the lowest BID). If there are multiple ports with the same BPDU, then it means that there is a redundant path to the root switch. In that case, eliminate all the ports that do not receive a BPDU from the lowest BID and go to step 2.

2. Choose the port with the lowest Root Path Cost to the bridge. If there are multiple ports that have the same item, this means that the backup route has the same capacity/link speed as the primary route. Go to step 3.

3. Choose the port with the lowest Sender BID. If it is still the same, then go to step 4.

4. Choose the lowest port ID.

That is how a root port is chosen.