A non-designated port is an STP port that blocks traffic to prevent Layer 2 loops. Each segment has exactly one designated port; remaining ports become non-designated. In RSTP, this role is divided into alternate and backup roles. An alternate port provides a standby path toward the root bridge and can transition to forwarding if the root port fails. section 14.3.3 section 15.2.3
Determine the non-designated side of a remaining segment in this order:
- Identify ports that are already designated: all ports on the root bridge and ports connected to root ports qualify. section 14.3.3
- Compare the switches' root costs through their root ports. The switch with the lower root cost gets the designated port; the other port becomes non-designated. section 14.3.3
- Use bridge ID as the tiebreaker when root costs match. The higher-BID side becomes non-designated. section 14.3.3 section 14.6
- Expect the STP non-designated port to remain blocking; under RSTP, classify the equivalent standby role as alternate unless the same switch owns the designated bridge. section 14.3.3 section 15.2.3
Reading RSTP output
In `show spanning-tree`, an alternate port appears with role `Altn` and status `BLK`. For example, SW1 G0/1 is an alternate port while SW1 G0/0 is the root port. On a switch with two ports connected to the same segment, the equivalent port on the designated bridge can instead appear as `Back` for backup. section 15.2.3
