• Removes tie between VLANs and Spanning-Tree instances.
  • Needed when reaching the max limit of VLANs, because the switch will not support that many STP instances.
  • MST Region:
    • Bridges that all agree upon instance name, revision number, and VLAN to STP instance mappings.
    • Intra Region
      • Within Region, details are known.
      • VLAN to STPIs are manually defined.
      • Undefined VLANs fall into CIST/MST0
    • Inter Region
      • Information between regions is not known.
      • Regions see each other as virtual bridges.
      • Simplifies Inter-Region calculation.
      • Intra-Region MSTIs collapsed into CIST.
  • Configuration:
    • Requires Region name, Revision Number, VLAN to instance mapping
    • Enable MST globally.
    • Commands:
      • ‘spanning-tree mst <instance> priority’
      • ‘spanning-tree mst <instance> root <primary/secondary>’
      • ‘show spanning-tree mst <instance>’
      • ‘show spanning-tree root’
      • Enabling:
        • ‘spanning-tree mode mst’
        • ‘spanning-tree mst configuration’
        • ‘name REGION0’
        • ‘instance 1 vlan 1-10’
        • ‘instance 2 vlan 11-20’
    • The root bridge must be in the MST instance, otherwise instance will fail.
  • Backwards Compatibility:
    • When running RSTP or PVST alongside MST, a show spanning-tree will provide ‘Bound’ ports.
  • Ports Gi0/3 and Gi1/0 are connected to a device running PVST. This works though because the root bridge for VLAN 10 is in the MST region.
  • All commands are the same when using RSTP and MST, but instead of using ‘vlan’, we use ‘mst’ and region number/name.

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