Cisco Systems 3750E manual Disabling Spanning Tree, Configuring the Root Switch, 18-16

Chapter 18 Configuring STP

Configuring Spanning-Tree Features

Disabling Spanning Tree

Spanning tree is enabled by default on VLAN 1 and on all newly created VLANs up to the spanning-tree limit specified in the “Supported Spanning-Tree Instances” section on page 18-10. Disable spanning tree only if you are sure there are no loops in the network topology.

Caution When spanning tree is disabled and loops are present in the topology, excessive traffic and indefinite packet duplication can drastically reduce network performance.

Beginning in privileged EXEC mode, follow these steps to disable spanning-tree on a per-VLAN basis. This procedure is optional.

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	no spanning-tree vlan vlan-id	For vlan-id, the range is 1 to 4094.
Step 3
	end	Return to privileged EXEC mode.
Step 4
	show spanning-tree vlan vlan-id	Verify your entries.
Step 5
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To re-enable spanning-tree, use the spanning-tree vlan vlan-idglobal configuration command.

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-idroot 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 Table 18-1 on page 18-5.)

Note The spanning-tree vlan vlan-idroot 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.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
18-16	OL-9775-02