Configuring Path Cost - Cisco 3845 - Security Bundle Router Software Manual
Software configuration guide
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Chapter 14
Configuring MSTP
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
Step 1
configure terminal
Step 2
interface interface-id
Step 3
spanning-tree mst instance-id cost cost
Step 4
end
Step 5
show spanning-tree mst interface interface-id
or
show spanning-tree mst instance-id
Step 6
copy running-config startup-config
OL-23400-01
Purpose
Enter global configuration mode.
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.
If a physical interface is a UNI, before attempting to
Note
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
"Changing the Spanning-Tree Mode." section on
page
13-12.
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.
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.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Configuring MSTP Features
14-21
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