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Catalyst 2950 and Catalyst 2955 Switch Command Reference
0L-10102-01
Chapter 2 Catalyst 2950 and 2955 Cisco IOS Commands
spanning-tree mst root
spanning-tree mst root
Use the spanning-tree mst root global configuration command to configure the multiple spanning-tree
(MST) root switch priority and timers based on the network diameter. Use the no form of this command
to return to the default setting.
spanning-tree mst instance-id root {primary | secondary} [diameter net-diameter
[hello-time seconds]]
no spanning-tree mst instance-id root
Syntax Description
Defaults The primary root switch priority is 24576.
The secondary root switch priority is 28672.
The hello time is 2 seconds.
Command Modes Global configuration
Command History
Usage Guidelines Use the spanning-tree mst instance-id root command used only on backbone switches.
When you enter the spanning-tree mst instance-id root command, the software tries to set a high
enough priority to make this switch the root of the spanning-tree instance. Because of the extended
system ID support, the switch sets the switch priority for the instance to 24576 if this value will cause
this switch to become the root for the specified instance. If any root switch for the specified instance has
a switch priority lower than 24576, the switch sets its own priority to 4096 less than the lowest switch
priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value.)
instance-id Range of spanning-tree instances. 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 15.
root primary Force this switch to be the root switch.
root secondary Set this switch to be the root switch should the primary root switch fail.
diameter net-diameter (Optional) Set the maximum number of switches between any two end
stations. The range is 2 to 7. This keyword is available only for MST
instance 0.
hello-time seconds (Optional) Set the interval between hello bridge protocol data units
(BPDUs) sent by the root switch configuration messages. The range is 1 to 10
seconds. This keyword is available only for MST instance 0.
Release Modification
12.1(9)EA1 This command was introduced.
12.1(13)EA1 The value for the instance-id variable was changed.
12.1(20)EA2 This command was added to the standard software image (SI).
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