MST Instances Question!

[Elvin Arias]

Hello Cisco Support Community,    

This might be a MST silly question, but please reply.

Can the same switch be root bridge for the VLAN's in instance 0 and root bridge for the VLAN's on a other instance, for example: instance 1?

Thanks

[Thotsaphon Lueangwattanaphong · Accepted solution]

Hi,

  Sure it can.

F.e

!

spanning-tree mode mst

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spanning-tree mst configuration

name MSTP

revision 1

instance 1 vlan 10-20

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spanning-tree mst 1 root primary

spanning-tree mst 0 root primary

!

    Vlan10-20 is instance 1.  Other vlans is instance 0(default).

HTH,

Toshi

[Elvin Arias]

Thanks thotsaphon for you extremely fast reply

Now i will explain this scenario:

It's simply like this: I started a migration process from PVST to MST to have uniformity in my network. STP and the instance 0 was without any mapping with any VLAN's, i configured the new switches have other instance (instance 1) (the migration ones) and just identified something weird. The root bridge of the instance 0 is the root bridge for the VLANS of the instance 1, and the swithch of the instance 1 is the root for the VLANs int he instance 1 (of course). until now there's not any connectivity problem in the network but im afraid that something could happen.

Thanks.

[Thotsaphon Lueangwattanaphong]

Hi,

   I'm not sure about what your migration plan is. What you explained is possible. It can be that you still have PVST switches connected  MST switches(Migration). If MST switch receives PVST BPDU,it replicates the BPDU for instance 0 for every vlan configured on the port(trunk). This is a boundary port. MST switch is possibly elected to be a root of MST instance0(not vlans of instance 1). However,you are recommended to manually define a root bridge on MST switch for all vlans received on that trunk(boundary port) .

  You may want to read this : http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml

HTH,

Toshi