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STP forwarding delay

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  • 1.  STP forwarding delay

    Posted 01-24-2021 00:35

    In STP,  after a topology change, a port takes by default 30 seconds (2 x forwarding-delay) to move from the blocking to the forwarding state 

    What was the design purpose of this? Why would a port need to listen to BPDUs for 30 seconds before moving to the forwarding state? This would mean that given the default BPDU hello-time of 2 seconds, a blocking port needs to receive 15 BPDUs from each of the other ports on the same segment before it  can move to the forwarding state. This seems excessive.

    So, what was the reasoning for having a port wait so long before moving to the forwarding state after a topology change.


  • 2.  RE: STP forwarding delay
    Best Answer

    Posted 01-24-2021 01:55
    Yes, indeed this was excessive!

    That was quite reasonable in the old days, but technology has changed so much.
    That's why we now have RSTP, which doesn't use these timers.

    I put more information in your other thread:

  • 3.  RE: STP forwarding delay

    Posted 01-24-2021 02:11
    Thanks Luke.