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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP
Configuring MSTP Features
To configure a switch to become the root, use the spanning-tree mst instance-id root global
configuration command to modify the switch priority from th e defaul t 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 in stance to 24 576 if this
value will cause this switch 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 to4096 less than the lowest switch priority. (4096 is the value of the least -signif icant b it of
a 4-bit switch priority value as shown in Table17-1 on page 17-5.)
If your network consists of switches that both do 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 shoul d be a back bon e or dist ribution sw itch. Do no t
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 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 t hro ugh t he spanning-tree mst hello-time,
spanning-tree mst forward-time, and the spanning-tree mst max-age global configuration
commands.
Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch. This
procedure is optional.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 spanning-tree mst instance-id root primary
[diameter net-diameter [hello-time seconds]] Configure a switch as the root switch.
•For instance-id, you can specify a single instance, a range
of instances separated by a hyphen, or a series of instances
separated by a comma. The range is 0 to 4094.
•(Optional) For diameter net-diameter, specify the
maximum number of switches between any two end
stations. The range is2 to 7. This keyword is available
only for MST instance 0.
•(Optional) For hello-time seconds, specify the interval in
seconds between the generation of configuration messages
by the root switch. The range is 1 to 10 seconds; the
default is 2 seconds.
Step3 end Return to privileged EXEC mode.