1.1 - STP Root Bridge Election

The task for 1.1 includes: "Ensure that SW2 is the primary STP root bridge for VLAN105"

The SG shows spanning-tree vlan 105 root primary as the solution. Can I assume that setting SW2's STP priority to 0 for VLAN 105 would also be a valid solution?

I can't see why it wouldn't, but wanted to check.


  • Configuring priority zero on the Sw2 could also be the valid solution but if there are other switches too in the network, it's possible for Sw1 to loose the root bridge capability. So, it's always best practice to go for straight forward configuration like "root primary" or "root secondary".


  • Setting priority to zero is more deterministic than "root primary".

    The "root primary" is a kind of macro and works as follows:

    The spanning-tree root primary alters this switch's bridge priority to 24576. If you enter the spanning-tree root primary command and the switch does not become the root, then the bridge priority is changed to 4096 less than the bridge priority of the current bridge. The command fails if the value required to be the root bridge is less than 1. If the switch does not become the root, an error results.

    If you have another switch which is currently root with priority 0, you will receive an error message.

    I suggest that you answer this question with "root primary" or "priority %n" (whatever number is lower than the current root bridge) command and verify that it became the root bridge.

    But if you have additional requirement like "remains root if other swithces introduced later" or something in these lines - use "priority 0".

  • Thanks CTPAYC. I knew the "root primary macro was only a one time thing...but like you said, I guess it depends on the exact wording of the question at the time.

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