15-18
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
78-17058-01
Chapter 15 Configuring MSTP
Configuring MSTP Features
Configuring Path Cost
The MSTP path cost default value is derived from the media speed of an NNI. If a loop occurs, the MSTP
uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to
NNIs that you want selected first and higher cost values that you want selected last. If all interfaces have
the same cost value, the MSTP puts the interface with the lowest interface number in the forwarding state
and blocks the other interfaces.
Beginning in privileged EXEC mode, follow these steps to configure the MSTP cost of an interface. This
procedure is optional.
Note The show spanning-tree mst interface interface-id privileged EXEC command displays information
only for ports that are in a link-up operative state. Otherwise, you can use the show running-config
privileged EXEC command to confirm the configuration.
To return the interface to its default setting, use the no spanning-tree mst instance-id cost interface
configuration command.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify an interface to configure, and enter interface
configuration mode. Valid interfaces include physical NNIs and
NNI port-channel logical interfaces. The port-channel range is 1
to 48.
Note If a physical interface is not an NNI, enter the port-type
nni interface configuration command before you
configure MST path cost.
If the interface is a port channel, all members of the port
channel must be NNIs.
Step 3
spanning-tree mst instance-id cost cost Configure the cost.
If a loop occurs, the MSTP uses the path cost when selecting an
interface to place into the forwarding state. A lower path cost
represents higher-speed transmission.
• 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 15.
• For cost, the range is 1 to 200000000; the default value is
derived from the media speed of the interface.
Step 4
end Return to privileged EXEC mode.
Step 5
show spanning-tree mst interface interface-id
or
show spanning-tree mst instance-id
Verify your entries.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.
