Multiple-Instance spanning tree protocol (MSTP) ensures that only one active path exists between any two nodes in a spanning-tree instance. A spanning-tree instance comprises a unique set of VLANs, and belongs to a specific spanning-tree region. A region can comprise multiple spanning-tree instances (each with a different set of VLANs), and allows one active path among regions in a network.

Without spanning tree, having more than one active path between a pair of nodes causes loops in the network, which can result in duplication of messages, leading to a “broadcast storm” that can bring down the network.

Developed based on IEEE 802.1s, MSTP overcomes the limitations of STP, RSTP, and PVST. In addition to supporting rapid network convergence, it allows data flows of different VLANs to be forwarded along separate paths. This provides a better load sharing mechanism for redundant links.

MSTP provides the following features:

  • MSTP divides a switched network into multiple regions, each of which contains multiple spanning trees that are independent of one another.
  • MSTP supports mapping VLANs to spanning tree instances by means of a VLAN-to-instance mapping table. MSTP can reduce communication overheads and resource usage by mapping multiple VLANs to one instance.
  • MSTP prunes a loop network into a loop-free tree, which avoids proliferation and endless cycling of packets in a loop network. In addition, it supports load balancing of VLAN data by providing multiple redundant paths for data forwarding.
  • MSTP is compatible with STP and RSTP, and partially compatible with PVST.