Configuring Spanning Tree Bridge Priority

Refer to Figure Spanning Tree Topology. All switches must be in default configurations. After the topology stabilizes, switch A is elected as the 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.

Execute the following commands in the switch C.

Type the following:

Enter the Global Configuration Mode in ISS1.

iS5comm# configure terminal

Set the priority for the spanning tree protocol.

iS5comm(config)# spanning-tree priority 4096

For priority, the 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 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.

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

iS5comm# show spanning-tree

In Switch A

Root Id         Priority   32768

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

 Cost       200000

 Port       2 [Gi0/2]

Max age 20 sec 0 cs, forward delay 15 sec 0 cs

 Hello Time 2 sec 0 cs

MST00

Spanning Tree Protocol Enabled.

MST000 is executing the mstp compatible Multiple Spanning Tree Protocol

Bridge Id   Priority  32768

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

 Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs

Hello Time is 2 sec 0 cs

Name    Role       State        Cost     Prio   Type

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

Gi0/1   Alternate  Forwarding   200000   128    SharedLan

Gi0/2   Root       Forwarding   200000   128    SharedLan

In Switch B

Root Id         Priority 4096

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

 Cost       200000

 Port       2 [Gi0/2]

Max age 20 sec 0 cs, forward delay 15 sec 0 cs

 Hello Time 2 sec 0 cs

MST00

Spanning Tree Protocol Enabled.

MST000 is executing the mstp compatible Multiple Spanning Tree Protocol

Bridge Id   Priority  32768

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

 Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs

Hello Time is 2 sec 0 cs

Name    Role       State        Cost     Prio   Type

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

Gi0/1   Designated Forwarding   200000   128    SharedLan

Gi0/2   Root       Forwarding   200000   128    SharedLan

In Switch C

Root Id         Priority 4096

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

 Cost       0

 Port       0 [0]

Max age 20 sec 0 cs, forward delay 15 sec 0 cs

 Hello Time 2 sec 0 cs

MST00

Spanning Tree Protocol Enabled.

MST000 is executing the mstp compatible Multiple Spanning Tree Protocol

Bridge Id   Priority 4096

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

 Max age is 20 sec 0 cs, forward delay is 15 sec 0 cs

Hello Time is 2 sec 0 cs

Name    Role       State        Cost     Prio   Type

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

Gi0/1   Designated Forwarding   200000   128    SharedLan

Gi0/2   Designated Forwarding   200000   128    SharedLan

Execute the no spanning-tree priority from the Global Configuration mode command to set the Priority to its default value.

iS5comm(config)#no spanning-tree priority