Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 10 Configuring MSTP
Configuring MSTP Features
To configure the router to become the root switch, use the spanning-tree mst instance-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 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 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 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 9-1 on page 9-4.)
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 should be a backbone or distribution switch. Do not
configure an access switch as the spanning-tree primary root.