Change the VLAN-to-MSTI mapping effect

Question:

We are about to migrate a Cisco Catalyst 3560 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.

Answer:

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 Cisco 3560 Switch on the MST region.