3-23
Multiple Instance Spanning-Tree Operation
Configuring MSTP
Syntax: spanning-tree priority < priority-multiplier >
Every switch running an instance of MSTP has a Bridge
Identifier, which is a unique identifier that helps distinguish
this switch from all others. The switch with the lowest Bridge
Identifier is elected as the root for the tree.
The Bridge Identifier is composed of a configurable Priority
component (2 bytes) and the bridge’s MAC address (6 bytes).
The ability to change the Priority component provides
flexibility in determining which switch will be the root for the
tree, regardless of its MAC address.
This command sets the switch (bridge) priority for the
designated region in which the switch resides. The switch
compares this priority with the priorities of other switches in
the same region to determine the root switch for the region.
The lower the priority value, the higher the priority. (If there
is only one switch in the region, then that switch is the root
switch for the region.) The root bridge in a region provides the
path to connected regions for the traffic in VLANs assigned to
the region’s IST instance. (Traffic in VLANs assigned to a
numbered STP instance in a given region moves to other
regions through the root switch for that instance.)
The priority range for an MSTP switch is 0-61440. However,
this command specifies the priority as a multiplier (0 - 15)
of 4096. That is, when you specify a priority multiplier value
of 0 - 15, the actual priority assigned to the switch is:
(priority-multiplier) x 4096
For example, if you configure “2” as the priority-multiplier on
a given MSTP switch, then the Switch Priority setting is 8,192.
Note: If multiple switches in the same MST region have the
same priority setting, then the switch with the lowest MAC
address becomes the root switch for that region.
Syntax: spanning-tree trap errant-bpdu
Enables SNMP traps for errant-BPDUs. Note that this
command is designed to be used in conjunction with the
spanning-tree bpdu-filter command (see page 3-28) and
bpdu-protection command (see page 3-29).
The no form of the command disables traps on the switch.
(Default: Disabled.)
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