Configuring Spanning Tree Path Cost

Refer to Figure Spanning Tree Topology. All switches are configured for STP compatible using spanning-tree compatibility stp in Global Configuration mode. After the topology stabilizes and Switch A is elected as Root, the ports of all switches except Port 2 of Switch C are in forwarding state. Port 2 of Switch C is an alternate port, and it is in discarding state.

When a loop occurs in the network topology, spanning tree protocol may use path cost to determine the spanning tree states of the ports. Path cost is obtained from the speed of the interface. A user can configure lower path cost for an interface, if the port needs to be selected first or the user can configure higher path cost if the port needs to be selected last for putting it to forwarding state.

Path cost is used to determine the topology only if the loop in the network cannot be resolved using only the Bridge IDs. If all ports have same path cost values, then the lowest numbered port is first put into forwarding state by spanning tree.

Execute the following commands in the Switch C.

Type the following:

Enter the Global Configuration Mode in ISS1.

iS5comm# configure terminal

Specify the interface for which the path cost is to be configured

iS5comm(config)# interface gigabitethernet 0/1

Valid interfaces include physical interfaces and port-channel logical interfaces (port-channel port-channel-number).

Configure the cost for the interface.

iS5comm(config-if)# spanning-tree cost 2000

Note:

Observation after configuring the Path Cost for port 1 in Switch C: Port 2 of Switch B is the Alternate Port and Port 2 of Switch C is a Designated Port.

Return to the Privileged EXEC mode.

iS5comm(config)#end

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

iS5comm# show spanning-tree

In Switch A

Root Id         Priority   32768

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

 Cost       200000

 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  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   Designated Forwarding   200000   128    SharedLan

Gi0/2   Designated Forwarding   200000   128    SharedLan

In Switch B

Root Id         Priority 4096

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

 Cost       200000

 Port 1 [Gi0/1]

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   Root       Forwarding   200000   128    SharedLan

Gi0/2   Alternate  Forwarding   200000   128    SharedLan

In Switch C

Root Id         Priority 32768

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

 Cost       2000

 Port 1 [Gi0/1]

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   Root       Forwarding   200000   128    SharedLan

Gi0/2   Designated Forwarding   200000   128    SharedLan

Execute the spanning-tree cost Interface Configuration mode command to set the default value of the Spanning Tree Path Cost.

iS5comm(config-if)#no spanning-tree cost