5.4 STP in the Campus Network
5.4.5 Modifying port priority by VLAN

You can set the port priority for a port on a per-VLAN basis. The port with the lowest priority value for a specific VLAN forwards frames for that VLAN. If all ports have the same priority value for a particular VLAN, the port with the lowest port number forwards frames for that VLAN.

This command is useful for distributing data across parallel paths. By default, if there is a parallel connection between two devices, Spanning Tree will block one of the links. Traffic from all VLANs would travel on one link, and one link would be used only as backup.

By changing the port priority for a specific group of VLANs, you can distribute the VLANs across the two links. In the example in Figure , Port 1/2 will forward for VLANs 100 to 105 and block for VLANs 106 to 110. The opposing Port 1/1 will forward for VLANs 106 to 110 and block for VLANs 100 to 105.

To change the port VLAN priority for a port on a set command-based switch, enter the set spantree portvlanpri mod-num/port-num priority [vlans] in privileged mode. Verify the port-VLAN priority setting by entering the show spantree [mod-num/port-num] command in privileged mode.

To change the port priority on a Cisco IOS command-based switch, you can simply enter the spanning-tree vlan [stp-list] port-priority port-priority command in interface configuration mode. The example in Figure illustrates the use of this command by setting the interface FastEthernet 0/3 with a priority of 10 for VLANs 1-10. This command is verified by using the show spanning-tree command.

Interactive Lab Activity  (Flash, 260 kB)
  In this activity, you will learn how to configure a Catalyst 4000 to become the root bridge for the network.