Cisco Catalyst 3560-X Software Configuration Manual page 567

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Table of Contents

Configuring STP

Configuring the Root Switch

The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge

ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For

each VLAN, the switch with the lowest bridge ID becomes the root switch for that VLAN.

To configure a switch to become 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. When you enter this command, the software checks the switch priority of the

root switches for each VLAN. Because of the extended system ID support, the switch sets its own

priority for the specified VLAN to 24576 if this value will cause this switch to become the root for the

specified VLAN.

If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own

priority for the specified VLAN to 4096 less than the lowest switch priority. (4096 is the value of the

least-significant bit of a 4-bit switch priority value as shown in

The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the

root switch is less than 1.

If your network consists of switches that both do and do not support the extended system ID, it is unlikely

that the switch with the extended system ID support will become the root switch. The extended system

ID increases the switch priority value every time the VLAN number is greater than the priority of the

connected switches running older software.

The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not

configure an access switch as the spanning-tree primary root.

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.

After configuring the switch as the root switch, we recommend that you avoid manually configuring the

hello time, forward-delay time, and maximum-age time through the spanning-tree vlan vlan-id

hello-time, spanning-tree vlan vlan-id forward-time, and the spanning-tree vlan vlan-id max-age

global configuration commands.

Table 1-1 on page

Catalyst 3750-X and 3560-X Switch Software Configuration Guide

Configuring Spanning-Tree Features

Troubleshooting

Catalyst 3750-x