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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Configuring a Root Switch
Configuring a Secondary Root Switch
You can set a secondary root switch on a VLAN when the switch is in PVST+ mode or on an MISTP
instance when the switch is in MISTP mode.
The
set spantree root
secondary
command reduces the bridge priority to 16,384, making it the probable
candidate to become the root switch if the primary root switch fails. You can run this command on more
than one switch to create multiple backup switches in case the primary root switch fails.
To configure a switch as the secondary root switch, perform this task in privileged mode:
This example shows how to configure the secondary root switch for VLANs 22 and 24:
Console> (enable)
set spantree root secondary 22,24 dia 5 hello 1
VLANs 22,24 bridge priority set to 16384.
VLANs 22,24 bridge max aging time set to 10 seconds.
VLANs 22,24 bridge hello time set to 1 second.
VLANs 22,24 bridge forward delay set to 7 seconds.
Console> (enable)
To configure a switch as the secondary root switch for an instance, perform this task in privileged mode:
This example shows how to configure the secondary root switch for an instance:
Console> (enable)
set spantree root secondary mistp-instance 2-4 dia 4
Instances 2-4 bridge priority set to 8192
VLInstances 2-4 bridge max aging time set to 14 seconds.
Instances 2-4 bridge hello time set to 2 seconds.
Instances 2-4 bridge forward delay set to 9 seconds.
Switch is now the root switch for active Instances 1-6.
Console> (enable)
Configuring a Root Switch to Improve Convergence
By lowering the values for the Hello Time, Forward Delay Timer, and Maximum Age Timer parameters
on the root switch, you can reduce the convergence time. For information on configuring these timers,
see the
“Configuring Spanning-Tree Timers on the Switch” section on page 7-49
.
Note
Reducing the timer parameter values is possible only if your network has LAN links of 10 Mbps or
faster. In a network with links of 10 Mbps or faster, the network diameter can reach the maximum value
of 7. With WAN connections, you cannot reduce the parameters.
Task
Command
Configure a switch as the secondary root switch.
set spantree root
[
secondary
]
vlans
[
dia
network_diameter
] [
hello
hello_time
]
Task
Command
Configure a switch as the secondary root switch
for an instance.
set spantree root
[
secondary
]
mistp-instance
instance
[
dia
network_diameter
]
[
hello
hello_time
]