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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
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 by using the spanning-tree mst hello-time,
spanning-tree mst forward-time, and the spanning-tree mst max-age global configuration commands
after configuring the switch as the root switch.
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 Catalyst 2950 or Catalyst 2955 switch that supports the extended system ID as
the secondary root, the spanning-tree 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. For Catalyst 2950 switches without the extended
system ID support (software earlier than Cisco IOS Release 12.1(9)EA1), the switch priority is changed
to 16384.
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 15.
• (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.