Cisco Systems 3750E manual Spanning-tree mst instance-id root primary, 19-18

Chapter 19 Configuring MSTP

Configuring MSTP Features

To configure a switch to become the root, use the spanning-tree mst instance-idroot global configuration command to modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the root switch for the specified spanning-tree instance. When you enter this command, the switch checks the switch priorities of the root switches. Because of the extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value will cause this switch to become the root for the specified spanning-tree instance.

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

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, which is available only for MST instance 0, 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.

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 mst hello-time, spanning-tree mst forward-time, and the spanning-tree mst max-ageglobal configuration commands.

Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch. This procedure is optional.

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	spanning-tree mst instance-idroot primary	Configure a switch as the root switch.
	[diameter net-diameter[hello-time seconds]]	• For instance-id, you can specify a single instance, a range
		of instances separated by a hyphen, or a series of instances
		separated by a comma. The range is 0 to 4094.
		• (Optional) For diameter net-diameter, specify the
		maximum number of switches between any two end
		stations. The range is 2 to 7. This keyword is available
		only for MST instance 0.
		• (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 seconds; the
		default is 2 seconds.
Step 3
	end	Return to privileged EXEC mode.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
19-18	OL-9775-02