Configure the Spanning Tree Switch Priority

See STP Topology Example for the topology for this procedure.

• Values for the instance range are 1-16.

• Values for the VLAN range are 1-4096.

• Values for the instance ID are 1-64.

1. Map VLANs to an MST instance and specify a configuration name for an MST region.

   SEFOS# configure terminal
   SEFOS(config)# spanning-tree mst configuration
   SEFOS(config-mst)# instance 1 vlan 1 
   SEFOS(config-mst)# name region1
   SEFOS(config-mst)# end

2. After the topology stabilizes, verify the MST information for the CIST instance or the specified MST instance.

   • The range for the instance ID is 1-64.

   • detail displays spanning tree MST instance-specific details.

   1. In switch 1, type.

      SEFOS# show spanning-tree mst 1
       
      ## MST01
      Vlans mapped:    1
      Bridge     Address 00:01:02:03:04:01    Priority 32768
      Root       Address 00:01:02:03:04:01    Priority 32768
      Root       this switch for MST01
      Interface Role        Sts         Cost     Prio.Nbr Type
      --------- ----         ---          ----     -------- ----
      Ex0/1     Designated  Forwarding  200000   128.1     Link Type is Shared
      Ex0/2     Designated  Forwarding  200000   128.2     Link Type is Shared

   2. In switch 2, type.

      SEFOS# show spanning-tree mst 1
       
      ## MST01
      Vlans mapped:    1
      Bridge     Address 00:02:02:03:04:01    Priority 32768
      Root       Address 00:01:02:03:04:01    Priority 32768
      Interface Role        Sts         Cost     Prio.Nbr Type
      --------- ----         ---          ----     -------- ----
      Ex0/1     Root        Forwarding  200000   128.1     Link Type is Shared
      Ex0/2     Designated  Forwarding  200000   128.2     Link Type is Shared

   3. In switch 3, type.

      SEFOS# show spanning-tree mst 1
       
      ## MST01
      Vlans mapped:    1
      Bridge     Address 00:03:02:03:04:01    Priority 32768
      Root       Address 00:01:02:03:04:01    Priority 32768
      Interface Role        Sts         Cost     Prio.Nbr Type
      --------- ----         ---          ----     -------- ----
      Ex0/1     Root        Forwarding  200000   128.1     Link Type is Shared
      Ex0/2     Alternate   Discarding  200000   128.2     Link Type is Shared

   4. In switch 3, type.

      SEFOS# configure terminal
      SEFOS(config)# spanning-tree mst 1 priority 4096

      • The instance ID range is 1 to 64.

      • The priority range is 0 to 61440 in increments of 4096. The default is 32768. The lower the number the more likely the switch will be chosen as the root switch. Valid priority values are multiples of 4096.

      • Spanning-tree priority xxx configures the priority in RSTP if RSTP is running, or configures the CIST priority if MSTP is running. The spanning-tree MST instance priority configures the priority in MSTI and is supported only if MSTP is running.

      • A fter configuring the switch Priority for switch 2, switch 2 becomes the root bridge. Port 2 of switch 3 becomes the root port and port 1 of switch 3 becomes the alternate port. Switch 1 becomes the designated bridge with port 1 as the root port and port 2 as the designated port.

   5. Complete the verification.

      SEFOS(config)# end

3. After the topology stabilizes, type this command in all the switches.
   1. In switch 1, type.

      SEFOS# show spanning-tree mst 1
       
      ## MST01
      Vlans mapped:    1
      Bridge     Address 00:01:02:03:04:01    Priority 32768
      Root       Address 00:02:02:03:04:01    Priority 4096
      Interface Role        Sts         Cost     Prio.Nbr Type
      --------- ----         ---          ----     -------- ----
      Ex0/1     Root        Forwarding  200000   128.1     Link Type is Shared
      Ex0/2     Designated  Forwarding  200000   128.2     Link Type is Shared

   2. In switch 2, type.

      SEFOS# show spanning-tree mst 1
       
      ## MST01
      Vlans mapped:    1
      Bridge     Address 00:02:02:03:04:01    Priority 4096
      Root       Address 00:02:02:03:04:01    Priority 4096
      Root       this switch for MST01
      Interface Role         Sts         Cost     Prio.Nbr Type
      --------- ----         ---          ----     -------- ----
      Ex0/1     Designated   Forwarding  200000   128.1    Link Type is Shared
      Ex0/2     Designated   Forwarding  200000   128.2    Link Type is Shared

   3. In switch 3, type.

      SEFOS# show spanning-tree mst 1
       
      ## MST01  
      Vlans mapped:    1 
      Bridge     Address 00:03:02:03:04:01    Priority 32768
      Root       Address 00:02:02:03:04:01    Priority 4096
      Interface Role       Sts         Cost     Prio.Nbr Type
      --------- ----       ---         ----     -------- ----
      Ex0/1     Alternate  Discarding  200000   128.1    Link Type is Shared
      Ex0/2     Root       Forwarding  200000   128.2    Link Type is Shared