Configuring Spanning Tree Bridge Priority

For Topology, refer to figure Spanning Tree Topology. All switches must be in default configurations. Start and enable PVRST+ on all switches. After the topology stabilizes, switch A is elected as Root since it has the least MAC address. All ports of all switches (except Port 2 of switch C) are in forwarding state. Port 2 of Switch C has been detected as an alternate port and is in discarding state.

1. Execute the following commands in the switch C.

   Type the following:

   • Enter the Global Configuration Mode.

   iS5comm# configure terminal

   • Set the priority for the STP.

   iS5comm(config)# spanning-tree mode pvrst

   iS5comm(config)# spanning-tree vlan 1 brg-priority 4096

   • Configure STP is per VLAN in PVRST+.

   For priority, the user can provide a value from 0 to 61440 in increments of 4096. The priority provided by the user is combined with VLAN ID to get actual Priority for the instance created for this VLAN. If no value is specified, then VLAN ID is combined with a default value of 32768. The lower the number, the more likely that the switch will be chosen as the Root Switch.

   Valid priority values are 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. All other values are rejected.

   • Return to the Privileged EXEC Mode.

   iS5comm(config)#end

   Note:

   Observation after configuring the bridge priority for Switch C. Switch C has been detected as the Root and Port 1 of Switch B is the Alternate Port.

2. View the spanning tree information by executing the following show command.

   Type the following:

   iS5comm# show spanning-tree vlan 1

   In Switch A

   Spanning-tree for VLAN 1

   Root Id         Priority   4097

   Address    00:03:02:03:04:01

   Cost       200000

   Port       2

   Hello Time 2 Sec, Max Age 20 Sec, Forward Delay 15 Sec

   Spanning Tree Enabled Protocol PVRST

   Bridge Id       Priority 32769

   Address 00:01:02:03:04:01

   Hello Time 2 sec, Max Age 20 sec, Forward Delay 15 sec

   Name   Role        State        Cost   Prio   Type

   ----   ----        -----        ----   ----   -----

   Gi0/1  Designated  Forwarding   200000 128    SharedLan

   Gi0/2  Root        Forwarding   200000 128    SharedLan

   In Switch B

   Spanning-tree for VLAN 1

   Root Id         Priority   4097

   Address    00:03:02:03:04:01

   Cost       200000

   Port       2

   Hello Time 2 Sec, Max Age 20 Sec, Forward Delay 15 Sec

   Spanning Tree Enabled Protocol PVRST

   Bridge Id       Priority 32769

   Address 00:01:02:03:04:01

   Hello Time 2 sec, Max Age 20 sec, Forward Delay 15 sec

   Name    Role        State        Cost    Prio   Type

   ----    ----        -----        ----    ----   -----

   Gi0/1   Alternate   Discarding   200000  128    SharedLan

   Gi0/2   Root        Forwarding   200000  128    SharedLan

   In Switch C

   Spanning-tree for VLAN 1

   Root Id         Priority   4097

   Address    00:03:02:03:04:01

   Cost       200000

   Port       2

   Hello Time 2 Sec, Max Age 20 Sec, Forward Delay 15 Sec

   Spanning Tree Enabled Protocol PVRST

   Bridge Id       Priority 4097

   Address 00:01:02:03:04:01

   Hello Time 2 sec, Max Age 20 sec, Forward Delay 15 sec

   Name    Role        State        Cost   Prio   Type

   ----    ----        -----        ----   ----   -----

   Gi0/1   Designated  Forwarding   200000  128    SharedLan

   Gi0/2   Designated  Forwarding   200000  128    SharedLan

   Note:

   Execute the no spanning-tree vlan <Vlan-Id> brg-priority Global Configuration mode command to set the priority to its default value.

   iS5comm(config)# no spanning-tree vlan <Vlan-Id> brg-priority