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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter14 Configuring STP
Configuring Spanning-Tree Features
Configuring a Secondary Root Switch
When you configure a 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 swit ch 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 multiple backup ro ot switches. U se
the same network diameter and hello-time values that you used w he n y ou co nfigured t he p ri mary 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 to the default setting, use the no spanning-tree vlan vlan-id root global configuration
command.
Configuring Port Priority
If a loop occurs, spanning tree uses the port priority when selecting an NNI to put into the forwarding
state. You can assign higher p riority v al ues (l ow er num erical v alu es) to NN Is tha t you wa nt sele cted f irst
and lower priority values (higher numerical values) to ones that you want selected last. If all NNIs have
the same priority value, spanning tree puts the NNI with the lowest interface 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 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.
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 14-14.
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.