STP Root Port Election Tutorial

A root port is the interface on a non-root switch that is “closest” to the root bridge — specifically, the port that receives the lowest-cost Bridge Protocol Data Unit (BPDU) coming from the direction of the root. Every non-root switch in a spanning-tree topology must elect exactly one root port, and that port always sits in the forwarding state.

The Root Port Election Process

Each non-root switch works through up to three criteria, in order, to decide which of its ports becomes the root port:

1. Lowest accumulated path cost to the Root Bridge.
2. Lowest Sender Bridge ID (used only if there is a tie on path cost).
3. Lowest Sender Port ID — itself made up of Port Priority followed by Port Number, compared in that order (used only if there is still a tie after step 2).

In other words, a switch only moves on to the next criterion if the previous one ends in a tie.

STP Path Cost by Link Speed