Configure the Bridge Priority

See STP Topology Example for the sample topology referenced in this section. All switches must have the default settings. After the topology stabilizes, switch 1 is elected as root since it has the lowest MAC address. All ports, except port 2 of switch 3, are in forwarding state. Port 2 of switch 3 is detected as an alternate port and is in discarding state.

In this example, after the bridge priority for switch 3 is set, switch 3 is detected as the root and port 1 of switch 2 to be the alternate port.

1. In Global Configuration mode, set the spanning tree priority in switch 3.

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

2. View the spanning tree information.
   1. In Switch 1, type.

      SEFOS# show spanning-tree
      Root Id         Priority   4096                Address    00:03:02:03:04:01                Cost       200000                Port       2 [Ex0/2]               Max age 20 Sec, forward delay 15 Sec
                      Hello Time is 2 SecMST00Spanning Tree Protocol Enabled.MST000 is executing the mstp compatible Multiple Spanning Tree ProtocolBridge Id       Priority  32768                Address  00:01:02:03:04:01               Max age is 20 sec, forward delay is 15 sec
                      Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is DisabledName      Role         State        Cost     Prio   Type----      ----         -----        ----     ----   ------Ex0/1     Designated   Forwarding   200000    128    SharedLanEx0/2     Root         Forwarding   200000    128    SharedLan

   2. In Switch 2, type.

      SEFOS# show spanning-tree
      Root Id         Priority   4096                Address    00:03:02:03:04:01                Cost       200000                Port       2 [Ex0/2]                Max age 20 Sec, forward delay 15 Sec                Hello Time is 2 SecMST00Spanning Tree  Protocol Enabled.MST000 is executing the mstp compatible Mutiple Spanning Tree ProtocolBridge Id       Priority  32768                Address  00:02:02:03:04:01               Max age is 20 sec, forward delay is 15 sec
                      Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is DisabledName      Role         State        Cost     Prio   Type----      ----         -----        ----     ----   ------Ex0/1     Alternate    Discarding   200000    128    SharedLanEx0/2     Root         Forwarding   200000    128    SharedLan

   3. In Switch 3, type.

      SEFOS# show spanning-tree
      Root Id         Priority   4096                Address    00:03:02:03:04:01                Cost       0                Port       0 [0]                This bridge is the root                Max age 20 Sec, forward delay 15 Sec
                      Hello Time is 2 SecMST00Spanning Tree  Protocol Enabled.MST000 is executing the mstp compatible Mutiple Spanning Tree ProtocolBridge Id       Priority  4096                Address  00:03:02:03:04:01                Max age is 20 sec, forward delay is 15 sec
                      Hello Time is 2 sec                Dynamic Path Cost is Disabled                Dynamic Path Cost Lag-Speed Change is DisabledName      Role         State        Cost     Prio   Type----      ----         -----        ----     ----   ------Ex0/1     Designated   Forwarding   200000    128    SharedLanEx0/2     Designated   Forwarding   200000    128    SharedLan