Use the diameter 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 Catalyst 6500 series switch 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 use the hello keyword to override the automatically calculated hello time.
To preserve a stable STP topology, we recommend that you avoid configuring the hello time, forward
delay time, and maximum age time manually after configuring the Catalyst 6500 series switch as the root
To configure a Catalyst 6500 series switch as the root bridge, perform this task:
Router(config)# spanning-tree vlan vlan_ID root
primary [diameter hops [hello-time seconds]]
Router(config)# no spanning-tree vlan vlan_ID
This example shows how to configure the Catalyst 6500 series switch as the root bridge for VLAN 10,
with a network diameter of 4:
Router# configure terminal
Router(config)# spanning-tree vlan 10 root primary diameter 4
Configuring a Secondary Root Bridge
When you configure a Catalyst 6500 series switch as the secondary root, the STP bridge priority is
modified from the default value (32768) so that the switch is likely to become the root bridge for the
specified VLANs if the primary root bridge fails (assuming the other network devices in the network use
the default bridge priority of 32768).
If the extended system ID is enabled, STP sets the bridge priority to 28672. If the extended system ID is
disabled, STP sets the bridge priority to 16384.
You can run this command on more than one Catalyst 6500 series switch to configure multiple backup
root bridges. Use the same network diameter and hello time values as you used when configuring the
primary root bridge.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Configures a Catalyst 6500 series switch as the root
bridge. The vlan_ID value can be 1 through 4094, except
reserved VLANs (see
Table 15-5 on page
Clears the root bridge configuration.
Exits configuration mode.
Configuring STP and IEEE 802.1s MST