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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 10 Configuring STP
Configuring Basic STP Features
Configuring STP Port Priority
If a loop occurs, STP uses the port priority when selecting an interface to put into the forwarding state.
You can assign higher priority values (lower numerical values) to interfaces that you want selected first
and lower priority values (higher numerical values) that you want selected last. If all interfaces have the
same priority value, STP puts the interface with the lowest interface number in the forwarding state and
blocks other interfaces. The priority range is 0 to 255; the default is 128.
Cisco IOS uses the port priority value when the interface is configured as an access port and uses VLAN
port priority values when the interface is configured as a trunk port.
Beginning in privileged EXEC mode, follow these steps to configure the STP port priority of an
interface:
Note The show spanning-tree interface interface-id privileged EXEC command displays information
only if the port is in a link-up operative state. Otherwise, you can use the show running-config
interface privileged EXEC command to confirm the configuration.
To return the interface to its default setting, use the no spanning-tree vlan vlan-id port-priority
interface configuration command. For information on how to configure load sharing on trunk ports by
using STP port priorities, see the “Load Sharing Using STP” section on page 9-29.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Enter interface configuration mode, and specify an
interface to configure.
Valid interfaces include physical interfaces and
port-channel logical interfaces (port-channel
port-channel-number).
Step 3
spanning-tree port-priority priority Configure the port priority for an interface that is an access
port.
For priority, the range is 0 to 255; the default is 128. The
lower the number, the higher the priority.
Step 4
spanning-tree vlan vlan-id port-priority priority Configure the VLAN port priority for an interface that is a
trunk port.
• For vlan-id, the range is 1 to 1005. Do not enter
leading zeros.
• For priority, the range is 0 to 255; the default is 128.
The lower the number, the higher the priority.
Step 5
end Return to privileged EXEC mode.
Step 6
show spanning-tree interface interface-id
or
show spanning-tree vlan vlan-id
Verify your entries.
Step 7
copy running-config startup-config (Optional) Save your entries in the configuration file.