The device maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting of the
device priority and the device MAC address, is associated with each instance. For each VLAN, the device with the lowest bridge
ID becomes the root switch for that VLAN.
To configure a device as the root for the specified VLAN, use the spanning-tree vlan
root global configuration command to modify the device priority from the default value (32768) to a significantly lower value.
When you enter this command, the software checks the device priority of the root devices for each VLAN. Because of the extended
system ID support, the device sets its own priority for the specified VLAN to 24576 if this value will cause this device to
become the root for the specified VLAN.
If your network consists of devices that support and do not support the extended system ID, it is unlikely that the device
with the extended system ID support will become the root device. The extended system ID increases the device priority value
every time the VLAN number is greater than the priority of the connected devices running older software.
The root device for each spanning-tree instance should be a backbone or distribution device. Do not configure an access device
as the spanning-tree primary root.
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of device hops between any two end stations
in the Layer 2 network). When you specify the network diameter, the device 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.
This procedure is optional.
After configuring the device as the root device, we recommend that you avoid manually configuring the hello time, forward-delay
time, and maximum-age time through the spanning-tree vlan
hello-time , spanning-tree vlan
forward-time , and the spanning-tree vlan
max-age global configuration commands.