Cisco Systems 3750E manual Enabling Root Guard, Enabling Loop Guard, 20-18

Chapter 20 Configuring Optional Spanning-Tree Features

Configuring Optional Spanning-Tree Features

Enabling Root Guard

Root guard enabled on an interface applies to all the VLANs to which the interface belongs. Do not enable the root guard on interfaces to be used by the UplinkFast feature. With UplinkFast, the backup interfaces (in the blocked state) replace the root port in the case of a failure. However, if root guard is also enabled, all the backup interfaces used by the UplinkFast feature are placed in the root-inconsistent state (blocked) and are prevented from reaching the forwarding state.

Note You cannot enable both root guard and loop guard at the same time.

You can enable this feature if your switch is running PVST+, rapid PVST+, or MSTP.

Beginning in privileged EXEC mode, follow these steps to enable root guard on an interface. This procedure is optional.

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	interface interface-id	Specify an interface to configure, and enter interface configuration mode.
Step 3
	spanning-tree guard root	Enable root guard on the interface.
		By default, root guard is disabled on all interfaces.
Step 4
	end	Return to privileged EXEC mode.
Step 5
	show running-config	Verify your entries.
Step 6
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable root guard, use the no spanning-tree guard interface configuration command.

Enabling Loop Guard

You can use loop guard to prevent alternate or root ports from becoming designated ports because of a failure that leads to a unidirectional link. This feature is most effective when it is configured on the entire switched network. Loop guard operates only on interfaces that are considered point-to-point by the spanning tree.

Note You cannot enable both loop guard and root guard at the same time.

You can enable this feature if your switch is running PVST+, rapid PVST+, or MSTP.

Beginning in privileged EXEC mode, follow these steps to enable loop guard. This procedure is optional.

	Command		Purpose
Step 1	show spanning-tree active		Verify which interfaces are alternate or root ports.
	or
	show spanning-tree mst
Step 2	configure terminal		Enter global configuration mode.
		Catalyst 3750-E and 3560-E Switch Software Configuration Guide
20-18			OL-9775-02