The
switch maintains a separate spanning-tree
instance for each active VLAN configured on it. A bridge ID, consisting of the
switch priority and theswitch 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 as the root for the specified VLAN,
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. 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.
![Note](https://www.cisco.com/content/dam/en/us/td/i/templates/note.gif) Note |
If your network
consists of
switches that support 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.
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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.
This procedure is optional.
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
spanning-tree vlan
vlan-id
hello-time ,
spanning-tree vlan
vlan-id
forward-time ,
and the
spanning-tree vlan
vlan-id
max-age global
configuration commands.