Diffrence between spanningtree primary and spantree priority

Unanswered Question
Oct 13th, 2008


I want to configure spanning tree on one my swtich as a primary for specific VLAN

spanning-tree vlan 1 root primary

spanning-tree vlan 1 priority 0

I hope with the above mentioned commands we can achive the saem. But what diffrence between these..



I have this problem too.
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Jon Marshall Mon, 10/13/2008 - 12:09


"spanning-tree vlan 1 root primary" means the switch will set it's own priority lower than any priority it sees from other switches so the value is not always the same - it's just going to be the lowest vlaue and a multiple of 4096.

"spanning-tree vlan 1 priority 0" explicitly sets the priority to be 0 and if there is no other switch with 0 then it will become root. Generally a good root value is 8192 and a secondart value 16384 but these will only work if all the other switches are using default values ie. you haven't modified them.

One of the reasons you wouldn't want to use second command "spanning-tree vlan 1 priority 0" is that it gives you nowhere to go. If you wanted to promote another switch to be the STP root you have no value below 0 and the "spanning-tree vlan 1 root primary" would never work because it can't go lower than 0.

Hope this makes sense.


sateeshk10 Mon, 10/13/2008 - 17:36

Thanks for ur reply..

Which one is suggestable in datacenter enveronment...



Jon Marshall Mon, 10/13/2008 - 22:18


Cisco best practice is to use the "spanning-tree vlan root primary" rather than explicitly set priority.

Having said that i have have always explicitly set the STP root and secondary values ie.

on root switch - 8192

on secondary root - 16384

If you are unsure of the current values it would make sense to use the "spanning-tree root primary" command. But in a data centre you should know each and every switches value.

Just make sure you don't mix and match with both commands as that could make troubleshooting rather difficult.



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