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?

  • Has any answer solved your question? Then please accept it or your question will keep popping up here forever. Please also consider voting for useful answers.
    – Zac67
    Dec 17, 2023 at 12:45

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, 2023 at 5:41
  • No forwarding is done when a port is blocked, neither unicast nor broadcast nor flooding.
    – Zac67
    Dec 17, 2023 at 12:44
  • Hi @Zac67. I am talking about the scenario where mac tables of both switches are empty. Let's take your comment as example where fa0/24 on cat2 is blocked after stp convergence. When an external traffic arrives on cat1, cat1 won't have any clue that fa0/24 on cat2 is blocked, right? Will it flood both the ports and the traffic would be dropped once it reaches cat2's fa0/24? Dec 19, 2023 at 14:41
  • Once a port is blocked, its MAC table entries are removed and stay removed.
    – Zac67
    Dec 19, 2023 at 14:49

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