Configuring A Primary Root Switch - Cisco WS-C6506 Software Manual

Chapter 7 Configuring Spanning Tree

Configuring a Primary Root Switch

You can set a root switch on a VLAN when the switch is in PVST+ mode or on an MISTP instance when
the switch is in MISTP mode. You enter the set spantree root command to reduce the bridge priority
(the value that is associated with the switch) from the default (32768) to a lower value, which allows the
switch to 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. The switch checks the bridge priority of the current root switches for
each VLAN. The bridge priority for the specified VLANs is set to 8192 if this value will cause the switch
to become the root for the specified VLANs. If any root switch for the specified VLANs has a bridge
priority lower than 8192, the switch sets the bridge priority for the specified VLANs to 1 less than the
lowest bridge priority. Because different VLANs could potentially have different root switches, the
bridge VLAN-priority chosen makes this switch the root for all the VLANs that are specified. If reducing
the bridge priority as low as 1 still does not make the switch the root switch, the system displays a
message.
Enter the set spantree root command on backbone switches or distribution switches only; do not enter
Caution
this command 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)
To configure a switch as the primary root switch for an instance, perform this task in privileged mode:
Task
Configure a switch as the primary root switch for
an instance.
This example shows how to configure the primary root switch for an instance:
Console> (enable) set spantree root 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)
OL-8978-04
Command
set spantree root [vlans] [dia network_diameter]
[hello hello_time]
Command
set spantree root mistp-instance instance [dia
network_diameter] [hello hello_time]
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Configuring a Root Switch
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