The
switch maintains a spanning-tree instance for
the group of VLANs mapped to it. A
switch ID, consisting of the
switch priority and the
switch MAC address, is associated with each
instance. For a group of VLANs, the
switch with the lowest
switch ID becomes the root
switch.
When you configure a
switch as the root, you 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 spanning-tree
instance. When you enter this command, the
switch checks the
switch priorities of the root
switches. Because of the extended system ID
support, the
switch sets its own priority for the
specified instance to 24576 if this value will cause this
switches to become the root for the specified
spanning-tree instance.
If any root
switch for the specified instance has a
switch priority lower than 24576, the
switch sets its own priority to 4096 less
than the lowest
switch priority. (4096 is the value of the
least-significant bit of a 4-bit
switch priority value. For more information,
select "Bridge ID,
Switch Priority, and Extended System ID" link
in Related Topics.
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.
Use the
diameter keyword, which is available only for MST
instance 0, 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
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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 mst hello-time,
spanning-tree mst forward-time, and the
spanning-tree mst max-age global configuration commands.
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