Spanning Tree Protocol


  • Configured globally: spanning-tree mode rstp | pvstp | stp
  • PVST/PVST+ enables a single instance of spanning tree for each vlan
  • Lowest priority is the root for that particular vlan (combination of Mac address and BID)
  • Root bridge is configured per vlan eg: spanning-tree vlan 10 root primary or spanning-tree vlan 10 root secondary
  • The root bridge can be configured by setting the lowest priority for that switch on that particular vlan eg: spanning-tree vlan 10 priority 4096
  • Port cost can be configured globally or per vlan eg: (config-int)#spanning-tree vlan 10 cost 15

MST 802.1s

  • MST groups vlans into regions
  • MST must be enabled globally just like RSTP/PVST with this command spanning-tree mode mst
  • MST is configured from MST configuration mode like the old vlan database mode
  • To enter MST configuration mode from global configuration mode: spanning-tree mst configuration
  • To participate in multiple spanning tree instances (MST) each switch must have the same MST name and MST revision number. The name and revision number determines which MST region the switch belongs to.
  • There can be a maximum of 16 spanning tree instances per MST region
  • IST (internal spanning tree) is the only instance that sends/receives BPDU’s and is always instance 0
  • show current displays the current MST configuration (command run from MST configuration mode)
  • show pending shows the changes that will be applied on exit (command run from MST configuration mode)
  • Setting the root bridge for MST spanning-tree mst vlan 10 root primary
  • Configuring the instance and adding vlans to that instance within a MST region
    • (config-mst)#instance 1 vlan 20-50
    • (config-mst)#instance 2 vlan 80-100

Leave a Reply