set spantree root Parameters 550

set spantree root Parameters
Parameter Definition
root Designation to change the switch to the root switch. The set spantree
root command changes the bridge priority from 32768 to 8192.
508 Chapter 16  Using Spanning Tree with VLANs
The following switch output is an example of using the set spantree root command:
Terry_4000> (enable) set spantree root 1-4 dia 2
VLANs 1-2 bridge priority set to 8192.
VLANs 1-2 bridge max aging time set to 10.
VLANs 1-2 bridge hello time set to 2.
VLANs 1-2 bridge forward delay set to 7.
Switch is now the root switch for active VLANs 1-4.
Terry_4000> (enable)
The set spantree root command tells the switch to change the bridge priority to 8192, which
automatically changes the switch to the root bridge. The 1-4 represents the VLANs for which the
STP will change the parameters, and the dia 2 is the network diameter. To figure the network
diameter, we simply counted the number of switches from the root, including the root bridge,
which in our example equals 2.
Notice the output after the command. The bridge priority was changed to 8192, the maximum
age time was changed to 10, hello time is still 2 seconds, and the forward delay was set
to 7 seconds. If the network diameter is set, the STP sets the timers to what it would consider
efficient for that size network.
You can verify your STP configuration with the show spantree command. If you type the
command show spantree with no parameters, it will show you the spanning tree configuration
secondary Designation to change the switch to a secondary root switch if the
primary fails. This automatically changes the bridge priority from a
default of 32768 to 16384.
vlan_list An optional command that changes the STP parameters on a specified
VLAN. If no VLAN is specified, then it changes only VLAN 1 by
default. You can change the parameters for VLANs 1–1005.
dia network diameter Another optional command that specifies the maximum number of
bridges between any two points where end stations attach. You can
set these parameters from 2 to 7. Figure the network diameter by
starting at the root bridge and counting the number of bridges in the
VLAN. The root bridge is 1, so if you have only one more switch, set
the network diameter to 2. This changes the timers in the VLAN to
reflect the new diameter.
hello hello_time An optional command that specifies in seconds the duration
between configuration messages from the root switch. You can set
this anywhere from 1 to 10 seconds (2 is the default).