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STP: For how long does the Root Bridge keep the TC bit set?

  • 1.  STP: For how long does the Root Bridge keep the TC bit set?

     
    Posted 01-23-2021 01:54
    Hi everyone.

    Happy new year!

    I had an  question about the TC bit advertised in a Root Bridge's STP BPDU. 

    I know that when the Root Bridge receives a TCN BPDU, it starts setting the TC bit in its config BPDUs to inform all the switches that there's been a topology change.

    My question is:   For how long does the Root Bridge keep setting the TC bit in its BPDUs?

    Thanks,
    Deepak


  • 2.  RE: STP: For how long does the Root Bridge keep the TC bit set?

     
    Posted 01-23-2021 02:20
    Hi

    The TC bit will be set by the root for a period of max_age + forward_delay seconds, which is 20+15=35 seconds by default.
    This result into all the downstream bridged reducing their mac age out timer to 15 seconds.



  • 3.  RE: STP: For how long does the Root Bridge keep the TC bit set?

     
    Posted 01-23-2021 02:30
    Got it. Thanks Ravi!