Spanning Tree MST

Spanning Tree MST works on the principal that your infrastructure doesn't really physically change, you simply setup a layer 2 path then associate vlans with that path.

You create regions/instances and associate vlans with the region.

Check your current VLANs.

Switch#
sh vlan brief

You will need to setup each switch individually with the following code:

Switch(config)#
spanning-tree mode mst
spanning-tree mst configuration
name CISCO
revision 1
instance 1 vlan 10-50,60
instance 2 vlan 70-100,110,130
show current ! this will show the current (blank) mst configuration
show pending ! this will show the changes to be commited on exit
exit


This will create two MST instances and put the named vlans in them.

Switch#
show span mst config

To set the root bridge on each MST instance use:

SwitchALPHA(config)#
spanning-tree mst 1 root primary
spanning-tree mst 2 root secondary

SwitchBETA(config)#
spanning-tree mst 1 root secondary
spanning-tree mst 2 root primary


Other commands

Switch#
show spanning-tree mst 1 detail
show spanning-tree interface vlan 20 detail

Troubleshooting

Some old IOS may have pre-ieee mst standard which when you do a 'sh spann' the type field is shown as
- Bound PVST
- Pre-STD-Rx

The normal/default is p2p.

Bound PVST means the switch has fallen back to slow legacy PVST.
Pre-STD-Rx means the neibour is running a pre-ieee cisco propriotary version of mst.

Upgrade the older switch or hard code on the interface configure:
spanning-tree mst pre-standard
This will turn off the automatic/detection which is not 100% reliable.

Ideally all mst switch interface types should be: p2p.

Clear Spanning Tree

Shutdown interfaces or clear the detected-protocols on EVERY switch.

Switch(config)#
interface range g0/1-2
shut
no shut

Switch#
clear spanning-tree detected-protocol