A designated port is an STP port in the forwarding state that points away from the root bridge. Each LAN segment has exactly one designated port, ensuring that traffic has one active path while redundant links are blocked to prevent Layer 2 loops. section 14.3.3 section 14.6
Select designated ports using these rules:
- Make every port on the root bridge designated because those ports point away from the root bridge. section 14.3.3
- Make the port connected to a non-root switch’s root port designated; the root port’s neighbor must remain forwarding. section 14.3.2
- For each remaining switch-to-switch segment, choose the port on the switch with the lowest root cost. section 14.3.3
- If root costs tie, choose the port on the switch with the lowest bridge ID (BID). section 14.3.3
- Place the other ports on that segment in the non-designated blocking state, preventing Layer 2 loops. section 14.3.3
Here, a segment means a Layer 1 connection between switches, such as a link or hub collision domain. The term can also describe a Layer 2 LAN or VLAN, or a Layer 3 subnet, depending on context. section 14.3.3
Root-cost and BID comparison
If SW1 advertises a root cost of 4 and SW2 advertises 8 on the same segment, SW1’s port becomes designated and SW2’s port becomes non-designated. If both advertise a cost of 4, compare BIDs; the lower BID wins.
