Cisco Systems 3560 Manual De Usuario

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 

.)

The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be 
the root switch is less than 1.

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 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.

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.

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

Enter global configuration mode.

spanning-tree vlan vlan-id root primary 
[diameter net-diameter [hello-time seconds]]

Configure a switch to become the root for the specified VLAN.

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-time seconds, 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.

Return to privileged EXEC mode.