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13-12
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 13 Understanding and Configuring STP
Configuring STP
Configuring a Secondary Root Switch
When you configure a switch as the secondary root, the spanning tree bridge priority is modified from
the default value (32,768) to 16,384. This means that the switch is likely to become the root bridge for
the specified VLANs if the primary root bridge fails (assuming the other switches in the network use the
default bridge priority of 32,768).
You can run 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 configuring the primary root switch.
Note We recommend that you avoid manually configuring the hello time, forward delay time, and maximum
age time after configuring the switch as the root bridge.
To configure a switch as the secondary root switch, perform this task:
This example shows how to configure the switch as the secondary root switch for VLAN 10, with a
network diameter of 4:
Switch# configure terminal
Switch(config)# spanning-tree vlan 10 root secondary diameter 4
VLAN 10 bridge priority set to 16384
VLAN 10 bridge max aging time set to 14
VLAN 10 bridge hello time unchanged at 2
VLAN 10 bridge forward delay set to 10
Switch(config)# end
Switch#
This example shows how to verify the configuration of VLAN 1:
Switch#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32768
Address 0003.6b10.e800
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address 0003.6b10.e800
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Status
---------------- ---- --- --------- -------- --------------------------------
Fa3/1 Desg FWD 19 128.129 P2p
Fa3/2 Desg FWD 19 128.130 P2p
Fa3/48 Desg FWD 19 128.176 Edge P2p
Switch#
Command Purpose
Step 1
Switch(config)# [no] spanning-tree vlan
vlan_ID
root secondary [diameter
hops
[hello-time
seconds
]]
Configures a switch as the secondary root switch.
You can use the no keyword to restore the defaults.
Step 2
Switch(config)# end
Exits configuration mode.