Refer to Figure Spanning Tree Topology. All switches are configured for STP compatible using the “spanning-tree compatibility stp” Global Configuration mode command. After the topology stabilizes, Switch A is elected as Root and all ports of all switches except Ports 2 and 3 (alternate, discarding) of Switch C are in forwarding state.
Switch A: VLAN 1 -10.0.0.1/255.0.0.0
Switch B: VLAN 1 – 10.0.0.2 /255.0.0.0
Switch C: VLAN 1 – 10.0.0.3/255.0.0.0
When a loop occurs in a network topology, spanning tree may use the value of port-priority of the ports to decide the port that must be put in the forwarding state.
Port priority is used to determine the topology only if the loop in the network cannot be resolved using the Bridge IDs or path-cost.
If higher priority (lower numerical value) is assigned to a port, it goes to forwarding first and when lower priority (higher numerical value) is assigned to a port, it goes to forwarding last. If all ports have same priority values, spanning tree puts the lowest numbered interface to forwarding and blocks all other interfaces
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 0/1Valid interfaces include physical interfaces and port-channel logical interfaces (port-channel port-channel-number).
- Configure the port priority for spanning tree.
iS5comm(config-if) # spanning-tree port-priority 32For priority, the range is 0 to 240 in increments of 16. The default is 128. The lower the number, the higher the priority.
Valid priority values are 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, and 240. All other values are rejected.
Observation after configuring the Port Priority for Port 3 in Switch A: Ports 1, 2 of Switch B are the Alternate Ports and Port 3 is the root port.
- Return to the Privileged EXEC mode.
iS5comm(config)#end- View the spanning tree properties by executing the following show command.
iS5comm# show spanning-treeIn 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
Gi0/3 Designated Forwarding 200000 32 SharedLan
In Switch B
Root Id Priority 32768
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 Designated Forwarding 200000 128 SharedLan
In Switch C
Root Id Priority 32768
Address 00:03:02:03:04:01
Cost 2000
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 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 Alternate Forwarding 200000 128 SharedLan
Gi0/2 Alternate Forwarding 200000 128 SharedLan
Gi0/3 Root Forwarding 200000 128 SharedLan
- Execute the no spanning-tree port-priority Interface configuration command to set the Spanning Tree Port Priority to its default value.
iS5comm(config-if)#no spanning-tree port-priority