Let's consider the below image:

enter image description here

Let's assume the following:

  1. Cat1 is the root bridge
  2. fa0/23 on cat2 is blocked after stp has converged

When a frame arrives at cat1 to be forwarded to cat2, how does Cat1 determine not to send the traffic via fa0/23, since as far as Cat1 is concerned, fa0/23 on cat1 is not blocked?

1 Answer 1


Usually the lower port ID (for same link metric) takes precedence, so most likely Fa0/24 would be blocked.

The algorithm runs like that: Cat2 recognizes Cat1 as root bridge on Fa0/23 and Fa0/24. It selects Fa0/23 as root port for lower port ID.

Accordingly, Fa0/23 on Cat1 becomes a designated port. Fa0/24 is neither root nor designated port, so it is blocked.

Actual frame forwarding is based on the switch's MAC address table. When Cat2 uses only Fa0/23 for sending frames towards Cat1, Cat1's MAC table shows all addresses only on Fa0/23, so that is used to forward frames to end nodes.

  • Thanks @Zac67. When Cat1's MAC address table is empty, will the traffic from CAT1 towards CAT2 be flooded on both ports, even though one of the ports on CAT2 is blocked? Nov 19 at 5:41

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