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Catalyst 2940 Switch Software Configuration Guide
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Chapter11 Configuring STP
Configuring Spanning-Tree Features
Configuring a Secondary Root Switch
When you configure a Catalyst 2940 switch that supports the extended system ID as the secondary root,
the switch priority is modified from the default value (32768) to 2867 2. Th e sw itch is th en li kely 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 conf igure m ultiple ba ckup root switc hes. Use
the same network diameter and hello-time values as you used when y ou co nfigured the p rima ry r oot
switch with the spanning-tree vlan vlan-id root primary global configuration command.
Beginning in privileged EXEC mode, follow these steps to configure a switch to become the secondary
root for the specified VLAN. This procedure is optional:
To return the switch to its default setting, use the no spanning-tree vlan vlan-id root global
configuration command.
Configuring the Port Priority
If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the
forwarding state. You can assign higher priority values (lower numerical values) to interfaces that you
want selected first and lower priority values (higher numerical values) that you want selected last. If all
interfaces have the same priority value, spanning tree puts the inte rface wi th the lo west inte rface number
in the forwarding state and blocks the other interfaces.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 spanning-tree vlan vlan-id root secondary
[diameter net-diameter [hello-time
seconds]]
Configure a switch to become the secondary 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 1005.
•(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 seconds; the default is
2 seconds.
Use the same network diameter and hello-time values that you used
when configuring the primary root switch. See the “Configuring the
Root Switch” section on page 11-12.
Step3 end Return to privileged EXEC mode.
Step4 show spanning-tree detail Verify your entries.
Step5 copy running-config startup-config (Optional) Save your entries in the configuration file.