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Multiple Instance Spanning-Tree Operation
Configuring MSTP
Syntax: [no] spanning-tree instance < 1..16 > vlan < vid [ vid..vid ] >
no spanning-tree instance < 1..16 >
— Continued —
Note: The valid VLAN IDs that you can map to a specified
MSTI are from 1 to 4094. The VLAN ID-to-MSTI mapping does
not require a VLAN to be already configured on the switch. The
MSTP VLAN enhancement allows you to preconfigure MSTP
topologies before the VLAN IDs associated with each instance
exist on a switch.
When you use preconfigured VLAN ID-to-MSTI topologies,
ensure that MSTP switches remain in the same region by
mapping all VLAN IDs used in the region to the same MSTIs
on each regional switch.
When you upgrade switch software to release 13.x.x and later,
the existing MSTP topology configuration is automatically
saved. All existing VLAN ID-to-MSTI assignments are
maintained on a switch for uninterrupted MSTP network
operation.
Syntax: spanning-tree instance < 1..16 > priority < priority-multiplier >
This command sets the switch (bridge) priority for the desig-
nated instance. This priority is compared with the priorities
of other switches in the same instance to determine the root
switch for the instance. The lower the priority value, the higher
the priority. (If there is only one switch in the instance, then
that switch is the root switch for the instance.) The IST
regional root bridge provides the path to instances in other
regions that share one or more of the same VLAN(s).
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 for the
specified MST instance is:
(priority-multiplier) x 4096
For example, if you configure “5” as the priority-multiplier for
MST Instance 1 on a given MSTP switch, then the Switch Priority
setting is 20,480 for that instance in that switch.
Note: If multiple switches in the same MST instance have the
same priority setting, then the switch with the lowest MAC
address becomes the root switch for that instance.
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