The switch maintains a separate spanning-tree instance for each
active VLAN configured on it. A bridge ID, consisting of the switch priority
and the switch MAC address, is associated with each instance. For each VLAN,
the switch with the lowest bridge ID becomes the root switch for that VLAN.
To configure a switch to become the root for the specified VLAN, use
global configuration command to modify the switch priority from
the default value (32768) to a significantly lower value. When you enter this
command, the software checks the switch priority of the root switches for each
VLAN. Because of the extended system ID support, the switch sets its own
priority for the specified VLAN to 24576 if this value will cause this switch
to become the root for the specified VLAN.
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 Table 14-1
on page 14-4 .)
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.
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
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
forward-time, and the
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.