10-23
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 10 Configuring STP
Configuring Basic STP Features
To configure a switch to become the root, use the spanning-tree vlan vlan-id root global configuration
command to modify the switch priority from the default value (32768) to a significantly lower value so
that the switch becomes the root switch for the specified VLAN. When you enter this command, the
switch checks the switch priority of the current 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 10-1 on page 10-3
.)
Note
The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to
be the root switch is less than 1.
Before Release 12.1(8)EA1, entering the spanning-tree vlan vlan-id root global configuration
command on a a Catalyst 3550 switch (no extended system ID) causes it to set its own switch priority
for the specified VLAN to 8192 if this value causes this switch to become the root for the specified
VLAN. If any root switch for the specified VLAN has a switch priority lower than 8192, the switch sets
its own priority for the specified VLAN to 1 less than the lowest switch priority.
These examples show the effect of the spanning-tree vlan vlan-id root command with and without the
extended system ID support:
•
For Catalyst 3550 switch with the extended system ID (Release 12.1(8)EA1 and later), if all network
devices in VLAN 20 have the default priority of 32768, entering the spanning-tree vlan 20 root
primary command on the switch sets the switch priority to 24576, which causes this switch to
become the switch bridge for VLAN 20.
•
For Catalyst 3550 switches without the extended system ID (software before Release 12.1(8)EA1),
if all network devices in VLAN 100 have the default priority of 32768, entering the spanning-tree
vlan 100 root primary command on the switch sets the switch priority for VLAN 100 to 8192,
which causes this switch to become the root switch for VLAN 100.
Note
If your network consists of Catalyst 3550 switches that do not support the extended system ID and
Catalyst 3550 switches that do support it, 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.
Note
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.
Use the 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 hello keyword to override the automatically calculated hello time.
Note
We recommend that you avoid manually configuring the hello time, forward-delay time, and
maximum-age time after configuring the switch as the root switch.
