Problems with Multiple Spanning Tree (MST)

[millerjord]

Hi gurus,

I have a simple problem, but can't see what I am doing wrong. I have a triangular setup between three 5406zl switches, and have made another mst instance to be able to allow some local VLAN's through a 10 gig line instead of a 2 gig trunk. See attached drawing.

Now I want to add another vlan to that instance. The vlan is numbered 200, and only exists in the two switches named h2sw001 and h2sw006. I tag the vlan on both switches on the interface D7, and everything works fine. Running the command "spanning-tree instance 1 vlan 200" on one of the switches works good, but when running the same command on the other switch the connection to that switch drops.
It doesn't matter what order I do it in, I lose connection with the last switch where I run that command. Management is in another VLAN in the ist instance. I don't have many VLANs configured on the switches (about 60), and no GVRP.

What am I missing?

[Richard Brodie_1]

I would do a "show span" before and after on the first switch, and see what is different in the configuration.

However, I think your problem is that  "spanning-tree instance 1 vlan 200" will change your MST configuration hash. You can't have difference VLAN to instance mappings in the same region: potentially you could have packets hop between different instance trees and cause a forwarding loop.

"show span mst-config" should show you.

I think "spanning-tree instance 1 vlan 200" on the third switch in the region should fix things, even if VLAN 200 doesn't actually reach it.