19-20
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter19 Configuring MSTP
Configuring MSTP Features
Configuring Port Priority
If a loop occurs, the MSTP 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, the MSTP puts the interface with the lowest interface number in the forwarding
state and blocks the other interfaces.
Note If your Catalyst 3750-E switch is a member of a switch stack, you must use the spanning-tree mst
[instance-id] cost cost interface configuration command instead of the spanning-tree mst [instance-id]
port-priority priority interface configuration command to select a port to put in the forwarding state.
Assign lower cost values to ports that you want selected first and higher co st values to ports that you
want selected last. For more information, see the “Configuring Path Cost” section on page19-21.
Beginning in privileged EXEC mode, follow these steps to configure the MSTP port priority of an
interface. This procedure is optional.
Note The show spanning-tree mst 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.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Specify an interface to configure, and enter interface
configuration mode.
Valid interfaces include physical ports and port-channel
logical interfaces. The port-channel range is 1 to 48.
Step3 spanning-tree mst instance-id port-priority priority Configure the port priority.
•For instance-id, you can specify a single instance, a
range of instances separated by a hyphen, or a series of
instances separated by a comma. The range is 0 to
4094.
•For priority, the range is 0 to 240 in increments of 16.
The default is 128. The lower the number, the higher
the priority.
The 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.
Step4 end Return to privileged EXEC mode.
Step5 show spanning-tree mst interface interface-id
or
show spanning-tree mst instance-id
Verify your entries.
Step6 copy running-config startup-config (Optional) Save your entries in the configuration file.