Cisco Systems N3KC3048TP1GE manual Configuring the Port Cost, Configuring the Switch Priority

Configuring Multiple Spanning Tree

Configuring the Port Cost

Configuring the Port Cost

The MST path cost default value is derived from the media speed of an interface. If a loop occurs, MST uses the 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 to interfaces values that you want selected last. If all interfaces have the same cost value, MST puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.

Note MST uses the long pathcost calculation method.

Procedure

	Command or Action	Purpose
Step 1	switch# configure terminal	Enters configuration mode.
Step 2	switch(config)# interface {{type	Specifies an interface to configure, and enters interface
	slot/port} {port-channel	configuration mode.
	number}}

Step 3 switch(config-if)# spanning-tree mst instance-idcost [cost auto]

Configures the cost.

If a loop occurs, MST uses the path cost when selecting an interface to place into the forwarding state. A lower path cost represents higher-speed transmission as follows:

•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 from 1 to 4094.

•For cost, the range is from 1 to 200000000. The default value is auto, which is derived from the media speed of the interface.

This example shows how to set the MST interface port cost on Ethernet 3/1 for MSTI 4:

switch# configure terminal switch(config)# interface ethernet 3/1 switch(config-if)# spanning-tree mst 4 cost 17031970

Configuring the Switch Priority

You can configure the switch priority for an MST instance so that it is more likely that the specified switch is chosen as the root bridge.

	Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
106	OL-26590-01