Spanning Tree Protocol Configuration Guide, Cisco Catalyst IE9300 Rugged Series Switches

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Spanning Tree Protocol Configuration Guide, Cisco Catalyst IE9300 Rugged Series Switches

Configure the root switch

Updated: March 2, 2026

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Learn how to configure a switch as the root for a specified VLAN by modifying its priority to ensure optimal network traffic flow and convergence.

You can configure a switch as the root for the specified VLAN by modifying the switch priority to a significantly lower value.

This task lets you control which switch becomes the root bridge for optimal network traffic flow and convergence. The switch priority is automatically set to 24576 if this value will make the switch the root.

Before you begin

To configure a switch as the root for the specified VLAN, use the spanning-tree vlan vlan-id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value.

Procedure

1.

Use the enable command to enter privileged EXEC mode.

Example:

Switch> enable

Enter your password if prompted.
2.

Use the configure terminal command to enter global configuration mode.

Example:

Switch# configure terminal
3.

Use the spanning-tree vlan root primary command to configure a switch to become the root for the specified VLAN.

Example:

Switch(config)# spanning-tree vlan 20 root primary diameter 4

When you enter this command, the software checks the switch priority of the root switches for each VLAN. With extended system ID support, the switch sets its own priority for the specified VLAN to 24576 if this value will make the switch the 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 commas. 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.

Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.
4.

Use the end command to return to privileged EXEC mode.

Example:

Switch(config)# end