STP root bridge priority with VLANs question

malcybood

Hi guys,

Currently learning Spanning-Tree inside out (to CCNA level for now) for my rematch with the 640-801

I think I've got the theory and gathering info about the root ID and bridge ID etc down, but I've got a quick querey on priorities that I can't find any info about quickly.

I have 3 switches A, B and C and have them trunked. On each switch there are ports assigned to 3 vlans across the VTP domain, VLAN 1, VLAN 10 and VLAN 20. I have set switch A as the root bridge by assigning VLAN 1 a priority of 4096, VLAN 10 = 8192 and VLAN 20 = 12,288. Having done this all 3 VLANs identify switch A as the root bridge which is what I want.

I initially assigned all 3 VLANs to having a priority of 4096 on 1, 10 and 20 whilst in a console connection my session started hanging and being very slow, hence why I changed the priorities to a different value for each VLAN.

So my question is, if you have several VLANs and you want to assign a particular switch as the root for all VLANS, do you have to assign a different priority number lower than 32768 for each VLAN on the root bridge?

Thanks

Malc

EdTheLad

No, each vlan is completely separate from the others so you can assign whatever priority you like.You will however have to assign a priority to the root switch in each vlan to be lower than the priority of any other switch in the respective vlan, that is the only requirement.If you have hundreds of vlans using the same root switch, the root will have to generate a bpdu for each separate vlan, this is alot of traffic and why MST was introduced.MST is a ccnp topic but if you want to learn more i just threw in the buzz word.
I dont know why you had issues with your connection, maybe a bug with the ios version?

malcybood

EdTheLad wrote:

No, each vlan is completely separate from the others so you can assign whatever priority you like.You will however have to assign a priority to the root switch in each vlan to be lower than the priority of any other switch in the respective vlan, that is the only requirement.If you have hundreds of vlans using the same root switch, the root will have to generate a bpdu for each separate vlan, this is alot of traffic and why MST was introduced.MST is a ccnp topic but if you want to learn more i just threw in the buzz word.
I dont know why you had issues with your connection, maybe a bug with the ios version?

Ed,

Great, thanks for clearing that up mate.

Not sure but when I pressed enter or the up/down arrow keys to get the previous commands, switch between user/priv (when entering passwords) it took 4 or 5 seconds to react then it reacted then it stalled for a second or 2 constantly through the session, when I changed the priorities to what I outlined in the post it was fine. I was consoled into the switch.

It wasn't my laptop either as I mentioned it was fine when I changed the priorities.

I've got 3 x 2950 switches with diff IOS versions so I'll try setting a different switch as the root for all VLANs with priorities being 4096 and ensure it works OK.

Thanks