KB.15.15

You must map at least one VLAN to an MSTI when you create it. You cannot map

a VLAN ID to more than one instance. You can create up to 16 MSTIs in a region.

The no form of the command removes one or more VLANs from the specified MSTI.

If no VLANs are specified, the no form of the command deletes the specified MSTI.

When you remove a VLAN from an MSTI, the VLAN returns to the IST instance,

where it can remain or be re-assigned to another MSTI configured in the region.

NOTE: Starting in software release 13.x.x, you can enter the spanning-tree

instance vlan command before a static or dynamic VLAN is configured on the

switch to preconfigure VLAN ID-to-MSTI mappings. No error message is displayed.

Later, each newly configured VLAN that has already been associated with an MSTI

is automatically assigned to the MSTI.

This new default behavior differs from automatically including configured (static

and dynamic) VLANs in the IST instance and requiring the manual assigning of

individual static VLANs to an MSTI.

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 using 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 upgrading 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.

Setting the bridge priority for an instance

Syntax:

spanning-tree instance 1..16 priority priority-multiplier

Sets the switch (bridge) priority for the designated instance. This priority is compared

with the priorities of other switches in the same instance to determine the root switch.

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 VLANs.

The priority range for an MSTP switch is 0 - 61440. However, this command

specifies the priority as a multiplier (0 - 15) of 4096. When a priority multiplier

value is set from 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, 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,

the switch with the lowest MAC address becomes the root switch for that instance.

Setting the bridge priority for an instance 91