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| 3.2 |  | STP Processes |
| 3.2.5 |  | Electing designated ports |

At this point, the Spanning-Tree Algorithm still has not eliminated any loops. This is taken care of after the Designated Ports and non-Designated Ports are determined. Each segment in a bridged network has one Designated Port. A Designated Port for a segment is the bridge port connected to that segment that both sends traffic toward the Root Bridge and receives traffic from the Root Bridge over that segment. The idea behind this is that if only one port handles traffic for each link, all the loops have been broken. The Designated Bridge for a segment is the bridge containing the Designated Port for that segment.

With the Root Port selection, the Designated Ports are chosen based on Root Path Cost to the Root Bridge. The Root Path Cost is the cumulative cost of all links leading to the Root Bridge. Segment 1 will form a link between Cat-A and Cat-B. This segment has two bridge ports, Port-1/1 on Cat-A and Port-1/1 on Cat-B. Port-1/1 on Cat-A has a Root Path Cost of 0 because Cat-A is the Root Bridge. Port-1/1 on CatA has a Root Path Cost of 0, because Cat-A is the Root Bridge. Port-1/1 on Cat-B has a Root Path Cost of 19, the value 0 received in BPDUs from Cat-A plus the path cost of 19 assigned to Port1/1 on Cat-B. Because Port-1/1 Cat-A has the lower Root Path Cost, it becomes the designated port for this link. For Segment 2, the Cat-A to Cat-C link, a similar election takes place. Port-1/2 on Cat-A has a Root Path Cost of 0, whereas Port-1/1 on Cat-C has a Root Path Cost of 19. Port-1/2 on Cat-A has the lower cost and becomes the designated port. Notice that every active port on the Root Bridge becomes a Designated Port.

Examining Segment 3, the link from Cat-B to Cat-C, both Port-1/2 on Cat-B and Port-1/2 on Cat-C have a Root Path Cost of 19, resulting in a tie. When faced with a tie, STP always uses the four-step decision sequence discussed in the “STP Decisions and BPDU Exchanges” section. Recall that the four steps are as follows:

Step 1 | Determine the Root Bridge
Step 2 | Calculate the lowest Root Path Cost
Step 3 | Determine the lowest sender BID
Step 4 | Determine the lowest Port ID

All three bridges, in Figure agree that Cat-A is the Root Bridge, so the algorithm checks Root Path Cost next. However, both Cat-B and Cat-C have a cost of 19, so the algorithm resorts to the lowest sender BID. Because the Cat-B BID (32,768.BB-BB-BB-BB-BB-BB) is lower than the Cat-C BID (32,768.CC-CC-CC-CC-CC-CC), Cat-B:Port-1/2 becomes the Designated Port for Segment 3. Cat-C:Port-1/2 becomes a non-Designated Port. Note that access ports do not play a role in the election of Designated Ports. The process described involves communication of STP parameters over trunk links.