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Configuring a Root Switch
To disable a MISTP instance, perform this task in privileged mode:
Task
Disable a MISTP instance.
This example shows how to disable a MISTP instance:
Console> (enable) set spantree disable mistp-instance 2
MI-STP instance 2 disabled.
Configuring a Root Switch
This section explains how to configure a primary root switch and a secondary root switch, and how to
prevent a switch from becoming a root switch using the root guard feature.
Configuring a Primary Root Switch
You can set a root switch on a VLAN when the switch is in PVST+ mode or on a MISTP instance when
the switch is in MISTP mode. Enter the set spantree root command to lower the bridge priority (the
value associated with the switch) below the default (32768); the switch can now become the root switch.
When you specify a switch as the primary root, the default bridge priority is modified so that it becomes
the root for the specified VLANs. Set the bridge priority to 8192. If this setting does not result in the
switch becoming a root, modify the bridge priority to be 1 less or the same as the bridge priority of the
current root switch. Because different VLANs could potentially have different root switches, the bridge
VLAN-priority chosen makes this switch the root for all the VLANs specified. If reducing the bridge
priority as low as 1 still does not make the switch the root switch, the system displays a message.
Caution
Enter the set spantree root command on backbone switches or distribution switches only, not on access
switches.
To configure a switch as the primary root switch, perform this task in privileged mode:
Task
Configure a switch as the primary root switch.
This example shows how to configure the primary root switch for VLANs 1–10:
Console> (enable) set spantree root 1-10 dia 4
VLANs 1-10 bridge priority set to 8192
VLANs 1-10 bridge max aging time set to 14 seconds.
VLANs 1-10 bridge hello time set to 2 seconds.
VLANs 1-10 bridge forward delay set to 9 seconds.
Switch is now the root switch for active VLANs 1-6.
Console> (enable)
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
7-28
Chapter 7
Command
set spantree disable mistp-instance instance [all]
Command
set spantree root [vlans] [dia network_diameter]
[hello hello_time]
Configuring Spanning Tree
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Troubleshooting
