[c-nsp] Changing the vlan-to-instance mapping for MST

[Dirk-Jan van Helmond]

We have an MST setup with 3 6500's, about 10 2960/2560s and +/- 20 Intel
Blade switches.

They all live in the same MST region and have identical MST name, revision
and instance-to-vlan mapping. Everything is converged nicely, no problem
there.

Now for arguments sake, suppose the instance-to-vlan mapping is as follows:

instance 1 vlan 1-20, 41-4093
instance 2 vlan 21-30
instance 3 vlan 31-40

Now i want to move a vlan from instance 1 to instance 3, i.e. vlan 50.

6500#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
6500(config)#spanning-tree mst configuration
6500(config-mst)#instance 3 vlan 50

now my instance mapping looks like:

instance 1 vlan 1-20, 41-49, 51-4093
instance 2 vlan 21-30
instance 3 vlan 31-40, 50



the problem is, because the instance-hash is not the same among all
switches, they regards eachother as different regions, effectively
converging in a way i don't want.

What is the best practice for changeing vlan-to-instance mappings?
Should I perform an semi-atomic change among all switches, accepting a
little downtime? Or can this be resolved by configuring MST0 (the
infrastructure MST) to converge in a similar way as MST 1 for instance?

thx!