topic Re: mstp about the spanning-tree priority parameter in Switches, Hubs, and Modems

mstp about the spanning-tree priority parameter

languedoc — Tue, 06 Jan 2009 22:48:26 GMT

Hi
Does the parameter spanning-tree priority has it's place in mstp configuration?

My config is like this:
Core1:
spanning-tree protocol-version MSTP
spanning-tree config-name "balance"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 1 2 3
spanning-tree instance 1 priority 0
spanning-tree instance 2 vlan 4 5 6
spanning-tree instance 2 priority 1
spanning-tree

Core2:
spanning-tree protocol-version MSTP
spanning-tree config-name "balance"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 1 2 3
spanning-tree instance 1 priority 1
spanning-tree instance 2 vlan 4 5 6
spanning-tree instance 2 priority 0
spanning-tree

But in the forum I have seen:

Core1:
spanning-tree priority 0
spanning-tree protocol-version MSTP
spanning-tree config-name "balance"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 1 2 3
spanning-tree instance 1 priority 0
spanning-tree instance 2 vlan 4 5 6
spanning-tree instance 2 priority 1
spanning-tree

Core2:
spanning-tree priority 1
spanning-tree protocol-version MSTP
spanning-tree config-name "balance"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 1 2 3
spanning-tree instance 1 priority 1
spanning-tree instance 2 vlan 4 5 6
spanning-tree instance 2 priority 0
spanning-tree

this secod config has an additional line for each switch "spanning-tree priority"

How will this line work on MSTP?
thanks

Re: mstp about the spanning-tree priority parameter

languedoc — Wed, 07 Jan 2009 16:23:53 GMT

hi
Any help please?
I'am sure someone knows.
Thanks

Re: mstp about the spanning-tree priority parameter

DaGuru — Thu, 08 Jan 2009 03:48:55 GMT

I believe the global priority is telling the network that Core1 and Core2 are roots for the Common Spanning Tree. In other words, these core switches would be recognized as the roots for switches running legacy STP versions.

I'm pretty sure this is done because the legacy STPs won't recognize the MST Instance info provided in the BPDUs.

If you have a test environment, this should be pretty easy to test; just put another switch into legacy opperation with and without the global priority set and observe the different behaviors. If your legacy switch has the same default priority as the core switches not configured with a global priority, the root election should be based on MAC. I would test this myself, but my equipment is currently tied up. Perhaps someone who knows for sure can post a reply.

Re: mstp about the spanning-tree priority parameter

DaGuru — Thu, 08 Jan 2009 03:52:08 GMT

I forgot to mention that with MSTP, dynamic VLANs always belong to the CST. So, you should see that static VLANs on the legacy switch will obey the root priority associated with the static VLANs that exist in your two MST instances.