Preconfiguring VLANs in an MST instance

When configuring an MSTP regional topology, multiple spanning tree instances are created. Each MST instance provides a fully connected active topology for a particular set of VLANs.

Each switch in an MSTP region is configured with the following set of common parameters:
  • Region name (spanning-tree config-name)

  • Region revision number (spanning-tree config-revision)

  • Identical VLAN ID-to-MSTI mapping (spanning-tree instance vlan)


spanning-tree instance 1..16 vlan vid [vid..vid]
no spanning-tree instance 1..16 vlan vid [vid..vid]

Configuring MSTP on the switch automatically configures the IST instance and places all statically and dynamically configured VLANs on the switch into the IST instance. This command creates a new MST instance (MSTI) and moves the VLANs specified from the IST to the MSTI.

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 removing a VLAN from an MSTI, the VLAN returns to the IST instance, where it remains or is re-assigned to another MSTI configured in the region.


The valid VLAN IDs to 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 preconfiguring 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.

Each MST instance supports a different set of VLANs. A VLAN that is mapped to an MST instance cannot be a member of another MST instance.

The MSTP VLAN configuration enhancement allows you to ensure that the same VLAN ID-to-MSTI assignments exist on each MSTP switch in a region. Before a static VLAN is configured or a dynamic VLAN is learned on the switch, use the spanning-tree instance vlan command to map VLANs to each MST instance in the region. Later, when the VLAN is created, the switch automatically assigns it to the MST instance to which you had previously mapped it.