To Next Page

To Previous Page

20-16

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

OL-21521-01

Chapter 20 Configuring STP

Configuring Spanning-Tree Features

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

sig

nificantly 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 p

riority 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

east-significant bit of a 4-bit switch priority value as shown in Table 20-1 on page 20-5.)

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

root switch is less than 1.

Note 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.

Note 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

iameter, 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.

Note 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.

Cisco Catalyst 3750-X User Manual

Other manuals for Cisco Catalyst 3750-X