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Help with Spanning Tree (MSTP?)

Matt Crossley
Occasional Visitor

Help with Spanning Tree (MSTP?)

Hi there,

I'm struggling with a good way to do this. Please see the attached image, which shows what I'm trying to do.

Basically, I have 2 VLANs, with switches 1,2,3 on VLAN 84, and all on VLAN 12.

What I'm finding is that when all 4 switches are connected together, I will get blocked traffic on VLAN84 from, say SW3. After realizing that it's a function of STP/RSTP, which doesn't understand VLANs, I realized I need to use MSTP.

After reading the documentation, I'm actually not sure about how to configure these switches for MSTP so that flow is blocked in the right places.

Can anyone point me in the right direction?

I'm thinking something like this:

SW1:
spanning-tree config-name "TEST"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 84
spanning-tree instance 1 priority 0
spanning-tree instance 2 vlan 12

SW2:
spanning-tree config-name "TEST"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 84
spanning-tree instance 2 vlan 12

SW3:
spanning-tree config-name "TEST"
spanning-tree config-revision 1
spanning-tree instance 1 vlan 84
spanning-tree instance 2 vlan 12

SW4:
spanning-tree config-name "TEST"
spanning-tree config-revision 1
spanning-tree instance 2 vlan 12
spanning-tree instance priority 0


If this is right, what I'm wondering about is what happens to the remaining STP and RSTP that exists for other VLANs that are attached to SW4? Will they be affected? Will I need to create regions for those other VLANs?

Any advice is much appreciated!
1 REPLY
Mohammed Faiz
Honored Contributor

Re: Help with Spanning Tree (MSTP?)

When dealing with MSTP it's best to simplify/standardise your configurations as much as you can.
I'd recommend that you run VLAN 84 to switch 4 also and have both VLANs tagged on all switch interlinks
Your config should be fine then and you'll have switch 1 as the root for instance 1 and switch 4 as the root for instance 2 (you've missed a '2' for config on switch 2).
With regards to the other VLANs any STP packets seen from other switches will be treated as a seperate region and only the vlans in the CIST will be forwarded.
The simplest way to deal with it is to enable bpdu-filter on the port that connects from switch 4 to the other switches (although that introduces a few risks but nothing too serious).