Multiple Spanning Tree Instances control the topology inside an MSTP region. An MSTI is a spanning tree domain that operates within a region and is bounded by that region; and MSTI does not exchange BPDUs or send notifications to other regions. You can map multiple VLANs to an MSTI; however, each VLAN can belong to only one MSTI. You can configure up to 64 MSTIs in an MSTP region.