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Cisco Systems Intelligent Gigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
Chapter 10 Configuring MSTP
Configuring MSTP Features
Configuring a Secondary Root Switch
When you configure a switch that supports the extended system ID as the secondary root, the
spanning-tree switch priority is modified from the default value (32768) to 28672. The switch is then
likely to become the root switch for the specified instance if the primary root switch fails. This is
assuming that the other network switches use the default switch priority of 32768 and therefore are
unlikely to become the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use
the same network diameter and hello-time values that you used when you configured the primary root
switch with the spanning-tree mst instance-id root primary global configuration command.
Beginning in privileged EXEC mode, follow these steps to configure a switch as the secondary root
switch. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst instance-id root global
configuration command.
Configuring the Port Priority
If a loop occurs, the MSTP uses the port priority when selecting an interface to put into the forwarding
state. You can assign higher priority values (lower numerical values) to interfaces that you want selected
first and lower priority values (higher numerical values) that you want selected last. If all interfaces have
the same priority value, the MSTP puts the interface with the lowest interface number in the forwarding
state and blocks the other interfaces.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
spanning-tree mst instance-id root
secondary [diameter net-diameter
[hello-time seconds]]
Configure a switch as the secondary 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 .
• (Optional) For diameter net-diameter, specify the maximum
number of switches between any two end stations. The range is 2
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.
Use the same network diameter and hello-time values that you used
when configuring the primary root switch. See the “Configuring the
Root Switch” section on page 10-13.
Step 3
end Return to privileged EXEC mode.
Step 4
show spanning-tree mst instance-id Verify your entries.
Step 5
copy running-config startup-config (Optional) Save your entries in the configuration file.