The device maintains
a separate instance of STP for each active VLAN in Rapid PVST+. For each VLAN,
the network device with the lowest bridge ID becomes the root bridge for that
To configure a VLAN
instance to become the root bridge, modify the bridge priority from the default
value (32768) to a significantly lower value.
When you enter the
command, the device checks the bridge priority of the current root bridges for
each VLAN. The device sets the bridge priority for the specified VLANs to 24576
if this value will cause the device to become the root for the specified VLANs.
If any root bridge for the specified VLAN has a bridge priority lower than
24576, the device sets the bridge priority for the specified VLANs to 4096 less
than the lowest bridge priority.
command fails if the value required to be the root bridge is less than 4096. If
the software cannot lower the bridge priority any lower, the device returns the
Error: Failed to set root bridge for VLAN 1
It may be possible to make the bridge root by setting the priority
for some (or all) of these instances to zero.
The root bridge
for each instance of STP should be a backbone or distribution device. Do not
configure an access device as the STP primary root.
keyword to specify the Layer 2 network diameter (that is, the maximum number of
bridge hops between any two end stations in the Layer 2 network). When you
specify the network diameter, the software automatically selects an optimal
hello time, forward delay time, and maximum age time for a network of that
diameter, which can significantly reduce the STP convergence time. You can
keyword to override the automatically calculated hello time.
With the device
configured as the root bridge, do not manually configure the hello time,
forward-delay time, and maximum-age time using the
forward-time , and
max-age global configuration commands.