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STP [Root Port Election] - Error in study guide?

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  • STP [Root Port Election] - Error in study guide?

    Hello all,

    I'm currently studying STP protocol, and understand how it work but according to the CCNA study guide (ie attached Thumbnails), I don't understand why answer C is wrong:

    I'm OK with Switch C will be the root bridge because of his lowest MAC address.

    Each "non root bridge" must elect a root port:
    Switch A: fa0/1 is the closest port to reach Root Bridge, so it is ROOT port
    Switch D: Gi0/2 is the closest port to reach Root Bridge, so it is ROOT port

    The problem is Switch C:
    1) Check Path Cost: SAME, next check
    2) Check Priority to neighboor switch: SAME, next check
    3) Check MAC adress: Switch A is lower thant switch D

    So Gi0/2 must be Root Port to reach Root Bridge (Switch B => Switch A => Switch C)

    Why this is wrong?

    Thanks
    Attached Files

  • #2
    Re: STP [Root Port Election] - Error in study guide?

    Its wrong because the path through switch D has the lower cost. There is a gig uplink from B to D with a cost of 4 by default. Then from D to C a fast ethernet trunk link which has a cost of 19 by default.

    B to A will autonegotiate as a fast ethernet trunk with a cost of 19 and the uplink from A to C is also 19.

    B - A - C = 38

    B - D - C = 23

    So gi0/2 on Switch B will be blocking. I think your confusion was that since B had a gig port it would be a gig uplink but the port connecting to A is only a fast ethernet port so it will negotiate to 100mb with a cost of 19. You have the concept down just look at the diagram and the question. Cisco can be tricky like that on exams. If you have the equipment lab it up which is the best way to see it in action.


    Here is a good link discussing RSTP

    http://www.cisco.com/en/US/tech/tk38...80094cfa.shtml
    Last edited by auglan; 3rd August 2010, 12:58. Reason: Added Link
    CCNA, CCNA-Security, CCNP
    CCIE Security (In Progress)

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    • #3
      Re: STP [Root Port Election] - Error in study guide?

      your confusion was that since B had a gig port it would be a gig uplink but the port connecting to A is only a fast ethernet port
      That's totally true, I didn't see that point, I understand now!

      Thanks

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