Change the VLAN-to-MSTI mapping effect

[mikik]

We are about to migrate a large network from  Rapid Spanning-tree to MST. There is one question , at the moment, which is not clear. Assuming that we have an MST region with 500 switches ( all in the same region).  We now wish to add one new vlan. What is the procedure and effect of this change.

My question relates to the process of the change. From my understanding , the moment I add one vlan to one switch, that switch will no longer be in the same region as the original region because the VLAN to MSTI mapping has changed. This will cause what effect. Where does one start to make this change? In the root bridge of the region, where then the moment the change is committed,  the old region won't see the former switches in its region?

If you start the change at the bottom, then as you make the change in more and more switches, they do will drop out of the original region. If all this is true then it seems that until all 500 switches have the changed MST configuration, there won't be a stable spanning-tree topology. This could create  a maitnance  issue where every such change may  take a long  time  and while the  change is being applied to all 500 switches the spanning-tree and hence the network will not be stable or production enabled.

Is this a real issue or is there something i am missing?

Thanks for any input

Mickey

[Peter Paluch · Accepted solution]

Hello Mickey,

Check out these threads:

https://supportforums.cisco.com/message/3501257#3501257

https://supportforums.cisco.com/thread/2107908#3458041

In short - creating or deleting a VLAN will not cause a topology change or a MST region separation because VLANs are pre-mapped into MST instances before they even exist. Changing this VLAN-to-instance mapping would indeed cause the MST region to become split until all switches were identically configured, but creating or deleting the VLAN alone has no impact on the MST region.

Best regards,

Peter

[mikik]

Hi Peter.

Thanks for the information. It seems clear that if the instance to vlan mapping does not change then adding vlans is none disruptive. I was referring to the case where the mapping did have to be modified. In this case I understand that there will be recalculations  and disruptions. As the client works now, only active vlans are defined and when they need a new vlan , they add it whereever they need. With MST it seems it would be wise to pre-map large groups of vlans so as to reduce disruptions. I hope I got it correct. If not feel free to correct me

Thanks for the help

Mickey

[Peter Paluch]

Hello Mickey,

I was referring to the case where the mapping did have to be modified.  In this case I understand that there will be recalculations  and  disruptions.

That is correct. Modifying the VLAN-to-instance mapping causes the MST configuration to be different until all switches have the updated configuration. During that time, the network will be split into several regions with possible transient outages.

With MST it seems it would be wise to pre-map large groups of vlans so as to reduce disruptions

Absolutely right.

Best regards,

Peter