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Quick STP question..

Discussion in 'Routing & Switching' started by The Editor, Jul 7, 2011.

  1. The Editor

    The Editor Bit Poster

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    OK I get that the root bridge is determined by the lowest bridge id.

    So my question is about which bridge gets elected root bridge. It's the lowest bridge id. 2 bytes for the priority and 6 for the MAC.

    Before the root bridge is determined what is held in the 2 byte priority field? Empty? And does that mean that the root bridge is basically the lowest MAC address?

    Thanks
    Bruce
     
    Certifications: CCNA
    WIP: CCNP, maybe CCNA SEC
  2. gosh1976

    gosh1976 Kilobyte Poster

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    The root priority field doesn't change unless you change it with a command like spanning-tree vlan1 4096. Also I don't believe it changes regardless of what switch is elected the root bridge. But, if you do a show-spanning tree you can see the brisge id of that switch and the root id which is the bridge id of the root bridge.

    I'm pretty sure that's right and might even make sense!
     
    Certifications: A+, Net+, MCDST, CCENT, MCTS: Win 7 Configuring, CCNA
  3. The Editor

    The Editor Bit Poster

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    So basically it is just lowest MAC that determines root bridge unless it is set by hand? Is that correct.

    Thanks
    Bruce
     
    Certifications: CCNA
    WIP: CCNP, maybe CCNA SEC
  4. gosh1976

    gosh1976 Kilobyte Poster

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    I believe so. Also, I think I left the word priority out of that command.
     
    Certifications: A+, Net+, MCDST, CCENT, MCTS: Win 7 Configuring, CCNA
  5. keconnect sparky

    keconnect sparky Nibble Poster

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    Thats correct.....

    In the root election that takes place to see who should be the root switch, the process looks at the BID (Priority + Mac Address) on the switches first (the part where the BPDU's probe the network to discover loops and elect a root bridge), the default priority is always 32768, so every switch is equal and due to this being a tie then looks at the MAC addresses on the switches,

    whoever is lower wins ..... however this does not always work out how you want it too, as the older switches always win this election as their MAC is always lower, so if you change the priority to say 1 (the lower the better), the election process will not have to look at the MAC's of the switches as the switch with this priority would have won therefore becoming the root switch.

    Changing the priority on a switch is a way for us network admins to rig the election to ensure the switch we want to come root, does :biggrin
     
    Certifications: MCP, CCENT, CCNA, CCNA-S
    WIP: CCNP (ROUTE)
  6. The Editor

    The Editor Bit Poster

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    Thanks. I was pretty sure that's now it worked just all the examples I saw had priorities set already. I just wanted to make sure that I understood the underlying concept.

    Appreciate it.
    Bruce
     
    Certifications: CCNA
    WIP: CCNP, maybe CCNA SEC

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