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10-27
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 10 Configuring STP
Configuring Basic STP Features
Configuring STP Path Cost
The STP path cost default value is derived from the media speed of an interface. If a loop occurs, STP
uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to
interfaces that you want selected first and higher cost values that you want selected last. If all interfaces
have the same cost value, STP puts the interface with the lowest interface number in the forwarding state
and blocks other interfaces.
STP uses the cost value when the interface is configured as an access port and uses VLAN port cost
values when the interface is configured as a trunk port.
Beginning in privileged EXEC mode, follow these steps to configure the STP cost of an interface:
Note The show spanning-tree 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 cost or the no spanning-tree vlan
vlan-id cost interface configuration command. For information on how to configure load sharing on
trunk ports using STP path costs, 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 cost cost Configure the cost for an interface that is an access port.
If a loop occurs, STP uses the path cost when selecting an
interface to place into the forwarding state. A lower path cost
represents higher-speed transmission.
For cost, the range is 1 to 200000000; the default value is derived
from the media speed of the interface.
Step 4
spanning-tree vlan vlan-id cost cost Configure the VLAN cost for an interface that is a trunk port.
If a loop occurs, STP uses the path cost when selecting an
interface to place into the forwarding state. A lower path cost
represents higher-speed transmission.
• For vlan-id, the range is 1 to 1005. Do not enter leading zeros.
• For cost, the range is 1 to 65535; the default value is derived
from the media speed of the interface.
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.