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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?

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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.

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  • 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?
    – host_unreachable
    Nov 19 at 5:41

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