fighting for the spanning-tree root bridge?

[james.chan.cisco]

Hi all, I don't have the switches so I couldn't do the test myself.

But as you know you can manipulate the root bridge priority by either put in:
sw1(config-if)#spanning-tree vlan 1 priority <0-61440> (the lower the better)
or
sw1(config-if)#spanning-tree vlan 1 root primary

so my question is, if you set a switch to have the lowest priority. Then go to another switch and issue the "root primary" command, will anything happen?

I doubt that I will "break" the spanning-tree but it'll be interesting to see...

[m.glosson · Accepted solution]

The "spanning-tree vlan X root primary" command is basically a "dynamic template" that will set the priority to either 24576 (if the current root is above priority 28672) or 4096 lower than the current root. Therefore, if the current root is, for example, 16384, the priority for the switch you issue that command on will become 12288 and it will take over root functions, which will cause the spanning-tree to recompute. Depending on which spanning-tree version you are running (PVST+, Rapid-PVST+, or MST) it will take a certain time period before all switches begin forwarding again.

If the current root switch already has the priority 0 and you type that command into a new switch, it will simply fail and give an error.

Cheers