INE R&S ATC019 - Multiple Spanning-Tree Protocol (MST)

MultipleSpanning-Tree Protocol

• IEEE (802.1s) response to PVST/PVST+
• Supports Rapid STP(802.1w)
• Instances are separate from VLANs
– PVST+ uses one instance per VLAN
– MST uses definable instances
• Highly scalable
– Switches with same instances, configuration revision number, and name form a “region”
– Different regions see each other as virtual bridges

MST Path Selection

• Same election process as CST/PVST
– Root bridge
• Lowest BID
– Root port
• Lowest cost
• Lowest upstream BID
• Lowest port ID

ChangingMST Root Bridge Election

• Manually change BID priority
– spanning-tree mst [instance] priority
– Lower is better
• Use root bridge macro
– spanning-tree mst [instance] root [primary | secondary]
– Sets local priority based on current Root Bridge
• Verification
– show spanning-tree mst [instance]
– show spanning-tree root

Changing an MST Port’s Role

• Modify the port’s cost
– spanning-tree mst[instance] cost
– bandwidth [bps]
• Modify the Bridge ID
– spanning-tree mst[instance] priority
• Modify the Port ID
– spanning-tree mst[instance] port-priority
• Verification
– show spanning-tree interface [int] detail
– show spanning-tree mst[instance] detail

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spanning-tree mst configuration
  name MST1
  revision 1
  instance 1 vlan 10,40,50
  instance 2 vlan 20,60,70
  instance 3 vlan 30,80,90
exit

spanning-tree mode mst
----------------------------------------------------------
show spanning-tree mst 1

int fa0/13
  spanning-tree mst 1 cost 1000000
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mst fastethernet port cost 200000
mst fastethernet port-channel port cost 100000