Thursday, 27 May 2010

MST - Multiple Spanning Tree - Don't change the mappings!

MST allows you to create spanning-tree instances and map VLANs into them.

Combined with VTP version 3 means you can advertise the MST mappings automatically, as shown here.

MST has the concept of regions.

Whether a switch is a member of a particular region depends on three things:

  • The configured MST region name.

  • The configured MST revision number.

  • The VLAN to MSTI mappings.

The entire VLAN to MSTI mapping isn't advertised in each BPDU but a checksum of the mapping table is.

So why do I care?

Because if you change the mappings, you change the region.

If you change the region that the root bridge belongs to, it's a topology change and you trigger a total STP re-convergence.

So if you're in a live network and you tweak the VLAN-to-MSTI mappings then you'll cause a complete outage. If it's running VTPv3 then the outage will be longer as the change ripples through the network and switches "re-join" the region.

The solution

The solution in this case is fairly easy, set up all your mappings on day 1 and stick to them!

If you've two switches then you're best off finding a suitable way to distribute the VLANs, e.g.

MSTI 1 - VLANS 1 to 1999 - Root bridge SW1
MSTI 2 - VLANS 2000 to 4096 - Root bridge SW2

Then make sure you set up HSRP/VRRP in future to match where the MSTI is rooted for each particular VLAN.

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