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  1. Only one comment:
    -Second scenary: BPDUs between SW2 and SW4: “Since SW2 temporarily claimed itself as the root bridge”, it should say “Since SW3 temporarily claimed itself as the root bridge”. Let me get to knoe im a mistake, thanks.


  2. Hi guys,

    two questions:

    1. When the root bridge looses a connection like in the example shown, why does it not send a TCN?
      Why only a TC trap is generated? It lost one of its designated ports. Or is that not a topology change from the roots perspective?

    2. When a new root port is elected on a switch and the new root port is the former designated port, there is now listening/learning/forwarding process happening, right? Because the port did that already and is already in a “forwarding” state!? Only if a port is in blocking state or the port just came UP, correct?


  3. Hello Florian

    Question 1
    The Root bridge never sends out TCNs because a TCN is used primarily to inform the root bridge of the topology change. More accurately, it is used to inform all switches between the topology change and the root bridge of the change. If a topology change occurs on the actual root bridge, it doesn’t need to send a TCN based on the above description. Note the following Cisco documentation:


    Question 2
    When a port changes from Designated po

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  4. Hi Laz,

    thanks a lot for your answer!

    1. I thought it might be faster for the root bridge to send a BPDU with the TC bit set right away when it realizes that a link is down, instead of waiting to hear from other switches in the network through a TCN and then sending the BPDU with the TC set.

    2. Got it

    I have two more questions.

    1. Just read the “Spanning-Tree UplinkFast” article and at one point the ND port changes to become the new root port, but it only goes through listening/learning/forwarding. I thought a ND equals a blocking port and thus needs to wait 20s

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  5. Hello Florian

    Concerning Question 1, if a root bridge learns about a topology change that is due to the change of state of its own interfaces, then it will immediately know of the topology change, so it does not actually have to “wait” to be informed by other switches. Once again, the TCN is sent to inform all the switches between the location where the topology change took place and the root bridge of the TC.

    Question 3:
    Yes you are correct that the ND port is indeed blocking. However, because its initial state is blocking, this state is not explicitly expre

    ... Continue reading in our forum

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