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Designated Port - Why is it (B) instead of (F)??

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  • Designated Port - Why is it (B) instead of (F)??

    Hi Guys,

    Any idea why SwitchA - Fa0/1 is elected as STP designated port instead of SwitchC - Fa0/1?

    What is being compared here?
    Attached Files

  • #2
    Re: Designated Port - Why is it (B) instead of (F)??

    I have a vague recollection (and it was almost 5 years ago) that it's to do with the mac address ?
    If they are all equal, then it sorts it out base on that?
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    • #3
      Re: Designated Port - Why is it (B) instead of (F)??

      Originally posted by tehcamel View Post
      I have a vague recollection (and it was almost 5 years ago) that it's to do with the mac address ?
      If they are all equal, then it sorts it out base on that?
      if it were to compare the MAC addresses, shouldn't switch C be elected as it has a lower MAC address?

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      • #4
        Re: Designated Port - Why is it (B) instead of (F)??

        wouldn't 0ace come before the face ?
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        • #5
          Re: Designated Port - Why is it (B) instead of (F)??

          I thought the same thing regarding the MAC address but the question is referring to the designated port, not the root bridge or root port. The MAC address would come in to play regarding root bridge election but I'm not sure how it factors in to the designated port selection.

          I don't work with Cisco stuff enough to know the answer, but I'm curious to find out.

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          • #6
            Re: Designated Port - Why is it (B) instead of (F)??

            OK, found the answer. It's similar to root bridge\root port selection. Int Fa0/0 on each switch are the root ports (ports with best path to root bridge). Since link Fa0\1 between switches A and C would create a loop, one of those ports has to be put in blocking mode. The switch with the lowest bridge ID wins and it's port is put into forwarding mode while the other switches port is put into blocking mode. Since switch A has the lower bridge ID (based on it's MAC address), and ignoring any other election parameters (since they're not included in the question) port Fa0\1 on switch A wins the selection.

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            • #7
              Re: Designated Port - Why is it (B) instead of (F)??

              Originally posted by joeqwerty View Post
              OK, found the answer. It's similar to root bridge\root port selection. Int Fa0/0 on each switch are the root ports (ports with best path to root bridge). Since link Fa0\1 between switches A and C would create a loop, one of those ports has to be put in blocking mode. The switch with the lowest bridge ID wins and it's port is put into forwarding mode while the other switches port is put into blocking mode. Since switch A has the lower bridge ID (based on it's MAC address), and ignoring any other election parameters (since they're not included in the question) port Fa0\1 on switch A wins the selection.
              Hi Joe,

              Thanks for the detailed explanation.

              But pardon my ignorance, given the 2 MAC:

              SwitchA - 0f00.cafe.1111

              SwitchC - 0f00.face.0000

              How do I compare thast SwitchA has a lower MAC add? By comparinf the 0f00. <c> and 0f00.<f> ?

              C is lower as compared to F, therefore SwitchA has lower brigde ID?

              Correct me if I am wrong.

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              • #8
                Re: Designated Port - Why is it (B) instead of (F)??

                AFAIK switch b is root because of lowest bridge id.
                since switch a and c are not roots and have a link between them which costs more to get to the root (if all links are equal it will take 2 links instead of 1 to the root) so the link a-c should be blocked.
                i am, however not sure which port would be set to "blocking"

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                • #9
                  Re: Designated Port - Why is it (B) instead of (F)??

                  Originally posted by kelvinwgr View Post
                  Hi Joe,

                  Thanks for the detailed explanation.

                  But pardon my ignorance, given the 2 MAC:

                  SwitchA - 0f00.cafe.1111

                  SwitchC - 0f00.face.0000

                  How do I compare thast SwitchA has a lower MAC add? By comparinf the 0f00. <c> and 0f00.<f> ?

                  C is lower as compared to F, therefore SwitchA has lower brigde ID?

                  Correct me if I am wrong.
                  you compare the same you would a decimal number.
                  cafe is smaller than face because f(15) is larger than c(12)

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                  • #10
                    Re: Designated Port - Why is it (B) instead of (F)??

                    Hi Guys,

                    I see. Thank you all for clearing my doubts. Cheers!

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