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Is there a particular reason for a Designated Port to continue sending out BPDUs after convergence has occurred on a link where the other side is in a STP blocking state? I get the idea of a Designated Port when a switch directly connects to the Root Port of another switch or when connecting to an edge device like a PC, but what's the purpose of determining a Designated Port between two switches when one side will inevitably enter into a blocking state to prevent a loop? Why don't both sides enter into a blocking state? Assigning a Designated Port in this case seems irrelevant. Is this just how the STP algorithm works?

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BPDUs are still sent because the switch topology could change, and it will then be detected. For example, if the root port on the switch with the blocked port either goes down or loses a path to the root in some other way, it will then look at the BPDUs being received on the blocked port and may decide that will now be its new root port.

Once STP has converged, that does not mean that things cannot change, otherwise you would need to restart everything when a change happens, or you could actually create a loop by making a change. BPDUs are continued to be sent to detect and adjust to any changes in the switch topology.

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  • What if the root port on the switch with the designated port goes down? What exactly happens to notify the other switch that the port it is currently blocking needs to now be a designated port since in a blocking state it would discard the BPDUs? Is it still able to process some overhead STP messages even though in a blocking state? – ahelton Nov 27 '19 at 15:21
  • When a topology changes, e.g. the root port goes down, the switch goes through the whole listening/learning procedure, and that is when it will hear the BPDUs on the port that had been blocked. It sounds like you need to review exactly how STP works, but this site is not really designed for a subject that large. Also, you forget that the designated port on the other switch could be serving users, too, so it is not blocked. Remember that ethernet was originally a shared medium, and that is when STP was adopted for it. – Ron Maupin Nov 27 '19 at 15:53
  • Should have clarified that I'm envisioning a three switch mesh design with SwA as root. I don't think my comment above clearly conveyed what I intended to ask. I understand that if a root port goes down on a switch (let's call it SwC), it's going to have to go through the whole STP process again on that particular switch. What I meant to ask was how does SwB, which currently has its port connected to SwC configured to block, know that it now needs to become a DP since SwC will need to use that particular Ethernet link as its new root (assuming SwC has no other paths to SwA to choose from)? – ahelton Nov 27 '19 at 17:03
  • It is because the BPDUs are being sent all the time, and STP will use those to reconverge. If you have a specific scenario, you could start a new question with that, but this is not a discussion forum, it is for specific questions and answers, so you should ask a new question for each question. STP is a very large subject, and comments like this lead to a discussion that is not appropriate for a Q&A forum. – Ron Maupin Nov 27 '19 at 17:06

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