PVSTP and different root bridges

Bl8ckr0uter

Greetings All:

Quick question: Could someone give me an example of a time when it would be a network good design choice to configure different Root bridges for different Vlans? Maybe I am missing something but this just doesn't make sense to me....

Also since you can do this, would this mean that the priorities would be different for each switch on a per vlan baises? Where do you configure this?

EdTheLad

If you had an edge switch with redundant links to two distribution switches.For load balancing you could config the distribution switches to be root for specific vlan ranges.You may also want to share out the processing load of the root among multiple switches rather than have one do all the work.You may not be happy to have a single point of failure for all vlans.
Under global config "spanning-tree vlan x priority" or "spanning-tree vlan x root primary"

Bl8ckr0uter

EdTheLad wrote: »

If you had an edge switch with redundant links to two distribution switches.For load balancing you could config the distribution switches to be root for specific vlan ranges.You may also want to share out the processing load of the root among multiple switches rather than have one do all the work.You may not be happy to have a single point of failure for all vlans.
Under global config "spanning-tree vlan x priority" or "spanning-tree vlan x root primary"

Interesting. I was reading in network warrior about this. They said it was possible but they didn't say why or when you would do this so I was curious.

Here is a question for you Ed:

Would it be possible to make seperate pieces of a switch (like the 3750s) in a stacked configuration or do they all keep the same priority in whatever vlan they are in?

Does this makes sense?

EdTheLad

A switch can support 4000+ vlans(some are reserved by the system hence you dont get 4096),for each of these vlans you can config a root bridge, for each port you can assign it to a vlan or make it a trunk.On each trunk you can allow all vlans or prune what you dont want.If you don't want the hassle of lots of vlan config and bpdus flowing everywhere you can configure mst.

So with all these options why would you want to seperate a switch into multiple pieces?

Bl8ckr0uter

EdTheLad wrote: »

A switch can support 4000+ vlans(some are reserved by the system hence you dont get 4096),for each of these vlans you can config a root bridge, for each port you can assign it to a vlan or make it a trunk.On each trunk you can allow all vlans or prune what you dont want.If you don't want the hassle of lots of vlan config and bpdus flowing everywhere you can configure mst.

So with all these options why would you want to seperate a switch into multiple pieces?

Like I am not sure how all of those switches are actually still switches when configuring vlans. Like when I was working on a 3750 the other day, the "switch" was actually several switches (8 I believe). I didn't get to play around with any vlan configs but If I wanted to could I have like switch 2 be the root bridge for 1 vlan and switch 4 being the root bridge for vlan 6?

The only reason why I ask is because I guess I dont know if those switches are actually treated as "switches".

Forsaken_GA

No, 3750's in a stack are treated logically as one switch, though they're seperate pieces of gear. It's one big switch with alot of ports.