In STP convergence process, I just know that after a link failed a blocking port become listening state for 15 seconds then learning state for 15 seconds. I've read a CCNA 200-301 Guide explained me that Listening state: The switch removes unused MAC address in MAC address table because these unused MAC entries can cause of potential loops. I still don't know how the loops appear if the switch skipped listening and learning state. I think when the link in topology failed that can't cause of the loops anymore. Can anyone give me an information about how the loops appear?

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    If a port moves from blocking directly to forwarding, it is not checking for a loop, and thus a loop could exist.
    – Ricky
    Jul 29, 2020 at 16:21

1 Answer 1


STP blocks redundant links in a network.

Generally, an Ethernet network is required to form a tree (of arbitrary breadth and depth). Any additional link between nodes in that tree creates a bridge loop, making broadcast frames circulate infinitely: A forwards to B forwards to C forwards to A forwards to B ... (plus causing other problems).

The STP algorithm locates and blocks these looping links. It creates a mathematical spanning tree where there is only a single path between any node pair. Any extra links are blocked.

When a new link is established, it might create a loop - you don't know yet. Accordingly, it is blocked for the time it takes to make sure that it doesn't create a loop - it is either integrated into the spanning tree and enters the forwarding state, or it is redundant and is blocked permanently.

If you have a port entering the forwarding state right after the link comes up (portfast or admin-edge mode) and it does create a loop, you'll have a broadcast storm raging for the time it takes for the spanning tree to reconverge. Likely, you wouldn't want that for a redundant link created on purpose.

  • Thank you for the answer. Could you give me some examples of some situations when temporary loop occurs if the redundant link change its state from blocking to forwarding immediately? Jul 30, 2020 at 7:29
  • When the link in the forwarding state creates a loop, the loop always causes a broadcast storm - see the answer above.
    – Zac67
    Jul 30, 2020 at 8:43

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