Migration from PVST to MST - Configuration Example...question...

[ehamil1984]

Hi Everyone,

First post in the community, yey!

Hopefully this will be a quick one to answer.  I am making preparations to migrate from PVST to MST on our network (as we are slowly but surely encroaching on the 128 VLAN limit).  All is going well with preparations by following the various bits of documentation and advisaries noted in this configuration example (http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml).  Most of it is self-explanatory and is as expected, however I'm scratching my head a bit on how best to interpret the following extract:

"Any MST configuration that involves a large number of either current           or new logical VLAN ports must be completed within a maintenance window because           the complete MST database gets reinitialized for any incremental change, such           as the addition of new VLANs to instances or the movement of VLANs across           instances."

At present, I can add/remove VLAN interfaces as seen fit and STP is not affected.  Does this extract suggest that this will no longer be the case?  Will adding/removig vlan interfaces from a switch configuration cause the database to be reinitialized once we have moved to MST?  Its the part where it mentions logical VLAN ports thats got me a bit consused at the moment.

I understand that adding/removing vlans themselves from an instsance will cause the database to be re-intialized, this much is obvious! I just want to be sure that MST wont see the removal of a VLAN interface as an incremental change.

Any input would be appreciated folks!  Thanks in advance!

ehamil1984

[Giuseppe Larosa · Accepted solution]

Hello Ewan,

your understanding is correct only when changing the Vlan to MST instance mapping there is a change.

The sentence that you have mentioned is followed by the following words:

>>such as the addition of new VLANs to instances or the movement of VLANs across instances.

What you do with L3 SVI interface does not matter to MST.

What is really interesting of MST implementation is that you can use provisioning that is you can associate to an instance Vlans that are not defined in the vlan database.

The suggestion from Cisco Expert Francois Tallet is to take advantage of this by dividing the 4094 Vlan space in N subsets and associating each subset to an MST instance.

When in the future you need a new Vlan you can pick up a vlan from the subset associated to the MST instance that has the desired topology without any changes to MST.

This is the different approach required by MST.

Hope to help

Giuseppe

[ehamil1984]

Hi Giuseppe,

Perfect, many thanks for the reply!  Puts my mind at ease!

The method you describe about provisioning within MST is also really useful - will definitely be of help in the event that future vlan changes are necessary!  I'll be sure to check it out!

Thanks again for your help!

Regards,

Ewan

[Purna Wijenayaka]

Hi ,

I am planning to migrate from PVST to MST , But I cannot change all the switches which participate in the domain ,

Is it possible to run PVST on some switches , and MST only on two switches out of the total switches which are  participating

in the STP domain ,

Please advice ,

Regards,

Purna