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.
MST uses the long path-cost calculation method.
Note
Procedure
Purpose
Command or Action
Enters global configuration mode.
switch#
configure terminal
Step 1
Specifies an interface to configure, and enters interface
configuration mode.
switch(config)#
interface
{{
type
slot
/
port
} | {
port-channel
number
}}
Step 2
If this is a 10G breakout port, the
slot
/
port
syntax is
QSFP-module
/
port
.
Note
Configures the cost.
switch(config-if)#
spanning-tree
mst instance-id cost
[
cost
|
auto
]
Step 3
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 6000 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
86
Configuring Multiple Spanning Tree
Configuring the Port Cost
