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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 14 Configuring MSTP
Configuring MSTP Features
Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network
diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2
network). When you specify the network diameter, the switch automatically sets an optimal hello time,
forward-delay time, and maximum-age time for a network of that diameter, which can significantly
reduce the convergence time. You can use the hello keyword to override the automatically calculated
hello time.
Note
After configuring the switch as the root switch, we recommend that you avoid manually configuring the
hello time, forward-delay time, and maximum-age time through the spanning-tree mst hello-time,
spanning-tree mst forward-time, and the spanning-tree mst max-age global configuration
commands.
Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch. This
procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst instance-id root global
configuration command.
Configuring a Secondary Root Switch
When you configure a switch with the extended system ID support as the secondary root, the switch
priority is modified from the default value (32768) to 28672. The switch is then likely to become the root
switch for the specified instance if the primary root switch fails. This is assuming that the other network
switches use the default switch priority of 32768 and therefore are unlikely to become the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use
the same network diameter and hello-time values that you used when you configured the primary root
switch with the spanning-tree mst instance-id root primary global configuration command.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
spanning-tree mst instance-id root primary
[diameter net-diameter [hello-time seconds]]
Configure a switch as the root switch.
•
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.
•
(Optional) For diameter net-diameter, specify 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.
•
(Optional) For hello-time seconds, specify the interval in
seconds between the generation of configuration messages
by the root switch. The range is 1 to 10 seconds; the
default is 2 seconds.
Step 3
end
Return to privileged EXEC mode.
Step 4
show spanning-tree mst instance-id
Verify your entries.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
