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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 '20 at 16:21
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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.

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  • 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? – Jarunchairat Wosonthia Jul 30 '20 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 '20 at 8:43

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