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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-07
Chapter 15 Configuring MSTP
Configuring MSTP Features
Configuring Path Cost
The MSTP path cost default value is derived from the media speed of an STP port. 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 STP ports 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.
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 or
ENIs with spanning tree enabled, VLANs, and NNI or ENI port
channels. The VLAN ID range is 1 to 4094. The port-channel
range is 1 to 48.
Note If a physical interface is a UNI, before attempting to
configure MST port priority, you must enter the port-type
nni interface configuration command or configure the
port as an ENI and enable spanning tree on the port. See
“Enabling Spanning Tree on an ENI” section on
page 14-13.
If the interface is a VLAN, only ports with spanning tree
enabled in the VLAN will run spanning tree.
If the interface is a port channel, all members of the port
channel must be NNIs or ENIs with spanning tree
enabled.
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 4094.
• 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.
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