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MSTP Questions

Unanswered Question
Aug 22nd, 2012
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It's my understanding that MSTP switches must have the following identical information configured in order for it to participate in a region:

  • Name
  • Configuration revision
  • VLAN-to-MSTI mapping

Do all VLANs need to be created on all switches in the region?  For instance, if I have two distribution layer switches and twelve access layer switches, each one with two connections to the distribution layer, do *all* VLANs defined in access switch #1 need to be defined in all access layer switches?  Or just the VLANs that are shared network-wide?  The specific situation I am trying to figure out is if I have an SVI (for routing) created in one switch, the VLAN for that SVI is never going to be used elsewhere in the network.  Does that VLAN need to be created everywhere or is it enough to just add it into its own instance which designates that switch as the root for that instance?

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krahmani323 Wed, 08/22/2012 - 12:57
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Hello evantol,

Basically the answer is no - You do not have to create all vlans on each switch particpating in the MST domain.

I would say it is all about the VLAN-to-MSTI mapping.

By default all the vlan (from 1 to 4094) are associated to instance 0 (named also IST).

Even if vlan 3000 (for ex.) is not created on the switch, it is mapped to instance 0 by default.

To change this "instanciation" the command is  =>  Switch(config-mst)# instance instance_id vlan vlan_range

So even if some vlans are not the same through different switches, one of the points that needs to match on all MST switches (region) is that VLAN-to-MSTI mapping.

(show spanning-tree mst configuration to verify)


Otherwise, in order to check that all the MST configuration is matching between bridges, I like to use the command show spanning-tree mst configuration digest which indicates the md5 digest calculated the name/config_revision/VLAN-to-MSTImapping.

This value should be the same between different switches to be considered as part of the same MST region.

If you need other information on how the MST acts on a trunk connecting two switches (based on the allowed vlans - each vlan being associated to its instance) please let us know !

Best regards.



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