Cisco Systems 3750E manual Configuring a Secondary Root Switch, Configuring Port Priority, 18-18

Chapter 18 Configuring STP

Configuring Spanning-Tree Features

Configuring a Secondary Root Switch

When you configure a switch as the secondary root, the switch priority is modified from the default value (32768) to 28672. The switch is then likely to become the root switch for the specified VLAN if the primary root switch fails. This is assuming that the other network switches use the default switch priority of 32768 and therefore are unlikely to become the root switch.

You can execute this command on more than one switch to configure multiple backup root switches. Use the same network diameter and hello-time values that you used when you configured the primary root switch with the spanning-tree vlan vlan-idroot primary global configuration command.

Beginning in privileged EXEC mode, follow these steps to configure a switch to become the secondary root for the specified VLAN. This procedure is optional.

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	spanning-tree vlan vlan-idroot secondary	Configure a switch to become the secondary root for the specified
	[diameter net-diameter[hello-time	VLAN.
	seconds]]	• 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 a comma. 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.
		• (Optional) For hello-timeseconds, specify the interval in
		seconds between the generation of configuration messages by
		the root switch. The range is 1 to 10; the default is 2.
		Use the same network diameter and hello-time values that you used
		when configuring the primary root switch. See the “Configuring the
		Root Switch” section on page 18-16.
Step 3
	end	Return to privileged EXEC mode.
Step 4
	show spanning-tree detail	Verify your entries.
Step 5
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return to the default setting, use the no spanning-tree vlan vlan-idroot global configuration command.

Configuring Port Priority

If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the forwarding state. You can assign higher priority values (lower numerical values) to interfaces that you want selected first and lower priority values (higher numerical values) that you want selected last. If all interfaces have the same priority value, spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.

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