Best way to set root bridge

[mwhite1983]

Hello,

A few questions regarding setting the root bridge...

What is the Cisco recommended way to set the root bridge for several vlans?

In our environment we will be deploying a collapsed core design with a stack of 4500X switches.  The access layer will consist of stacks of 2960X's (usually 1 stack per floor) directly connected to the core via fiber. 

In the core configs we currently have "spanning-tree vlan 1-999 root primary" to set on the core.  However, would it be better to specify individual vlans instead of having a blanket range of "1-999"?  We are not using all vlans 1-999 and have no plans to use all those vlans. 

Additionally, as an example, if we were to use "spanning-tree vlan 1, 10, 100,200,300,400,555 root primary" instead of "spanning-tree vlan 1-999 root primary", and needed to add an additional vlan, would adding the new vlan via "spanning-tree vlan XX root primary" cause RSTP to converge for all vlans or just the new vlan?

Thanks for any advice provided.

-Matt

[InayathUlla Sharieff]

Matt,

Yes two ways to do it:

1-

spanning-tree vlan 1, 10, 100,200,300,400,555 root primary

2-

spanning-tree vlan 1, 10, 100,200,300,400,555 priority 4096 .> by specifying priority value.

3- Question:

 add an additional vlan, would adding the new vlan via "spanning-tree vlan XX root primary" cause RSTP to converge for all vlans or just the new vlan?

Answer; it would be only one vlan.

HTH

Inayath

[lpember]

Hi mwhite1983,

I'm not qualified to give you an official Cisco recommendation for setting the root bridge for several VLANs (you'll want to refer to the Cisco configuration guides for that) but I can tell you that the configuration you currently have on your core should work fine.

Basically that configuration lowers the priority of your core to make it the root bridge on all of those VLANs. I don't believe using your blanket statement is any less efficient than specifying the VLANs you are actually using. If you add another VLAN to your network, the priority of the core should still be lower than all the other switches on the network and it will become the root bridge for that new VLAN. This means you shouldn't have to adjust any spanning tree settings when you add a new VLAN, though you may have to if you add a new switch thats priority is lower than than the default of 32,768.

Hope that helps.

[Joseph W. Doherty]

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Posting

You would normally want to align you root bridge with the VLAN gateway.

If you're running the 4500X in a VSS, with all VLAN gateways, it's probably easiest to make your VSS core the root for all VLANs.

I believe I've seen different switches use different priorities when you use the "root" parameter, so perhaps best to explictly define the root value.  You might want to use 8K for the normal root priority as it would allow you to override with 4K in needed.