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Catalyst 3750 Switch Software Configuration Guide
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Chapter 17 Configuring STP
Configuring Spanning-Tree Features
Use the diameter keyword 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 vlan vlan-id
hello-time, spanning-tree vlan vlan-id forward-time, and the spanning-tree vlan vlan-id max-age
global configuration commands.
Beginning in privileged EXEC mode, follow these steps to configure a switch to become the root for the
specified VLAN. This procedure is optional.
To return to the default setting, use the no spanning-tree vlan vlan-id root global configuration
command.
Configuring a Secondary Root Switch
When you configure a Catalyst 3750 switch 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
VLAN 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 vlan vlan-id root primary global configuration command.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
spanning-tree vlan vlan-id root primary
[diameter net-diameter [hello-time seconds]]
Configure a switch to become the root for the specified VLAN.
• For vlan-id, you can specify a single VLAN identified by
VLAN ID number, a range of VLANs separated by a
hyphen, or a series of VLANs separated by a comma. The
range is 1 to 4094.
• (Optional) For diameter net-diameter, specify the
maximum number of switches between any two end
stations. The range is 2 to 7.
• (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; the default is 2.
Step 3
end Return to privileged EXEC mode.
Step 4
show spanning-tree detail Verify your entries.
Step 5
copy running-config startup-config (Optional) Save your entries in the configuration file.
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