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Bridge Root Selection???

Discussion in 'Routing & Switching' started by albertc30, Feb 15, 2009.

  1. albertc30

    albertc30 Kilobyte Poster

    Hello everybody.
    It has been a while sense my last post, I have been really busy with my CCNA here at home but now that I was getting ready to undertake another exam, chapter 7, Spanning-Tree Protocol I got a question and I got it wrong on my practical quiz.
    If you guys please see the pic attached my answer to which switch will be the root bridge given the information???
    I thought that SW2 would be selected the root bridge as it has the lowest MAC address right? With the spanning-tree protocol is the reverse of the OSPF DR selection that is how I remember it, spanning-tree protocol root bridge selection is all about either the lowest bridge ID priority or MAC address no???
    Please have your say in the matter, your answer and as to why as I am a bit lost you guys and girls.
    Albert, C

    PS- I have changed the way the question was asked and so di do the diagram in PT also in a different way as to the one in the quiz so I guess I am ok when it comes to copyrights this time, hopefully.

    Certifications: CCNA
    WIP: 220-701 - A+
  2. albertc30

    albertc30 Kilobyte Poster

    The diagram would be of some help, :ops

    Attached Files:

    Certifications: CCNA
    WIP: 220-701 - A+
  3. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    The root bridge is the switch with the lowest priority. If two or more switches have the lowest priority, the root bridge is the switch with the lowest MAC.
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  4. keconnect sparky

    keconnect sparky Nibble Poster

    All tie-breaking STP decisions are based on;
    Step 1 - Lowest BID (Bridge ID aka MAC and Priority)
    Step 2 - Lowest Path Cost to Root Bridge
    Step 3 - Lowest Sender BID
    Step 4 - Lowest Port Priority
    Step 5 - Lowest Port ID

    So in your case SW3 is the root bridge

    Have a look at a previous post of mine , with some diagrams and outputs:
    Certifications: MCP, CCENT, CCNA, CCNA-S
  5. mark@ccierack.co.uk

    mark@ccierack.co.uk New Member

    Certifications: CNE, MCSE, CCNP

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