Multiple Spanning Tree Protocol

The MSTP (Multiple Spanning Tree Protocol), based on IEEE 802.1Q-2003 (formerly known as IEEE 802.1s), allows the bundling of multiple VLANs into one spanning tree topology.

This concept is not new to Extreme Networks. Like MSTP, Extreme Networks proprietary EMISTP implementation can achieve the same capabilities of sharing a virtual network topology among multiple VLANs; however, MSTP overcomes some of the challenges facing EMISTP, including enhanced loop protection mechanisms and new capabilities to achieve better scaling.

MSTP logically divides a Layer 2 network into regions. Each region has a unique identifier and contains multiple spanning tree instances (MSTIs). An MSTI (Multiple Spanning Tree Instances) is a spanning tree domain that operates within and is bounded by a region. MSTIs control the topology inside the regions. The Common and Internal Spanning Tree (CIST) is a single spanning tree domain that interconnects MSTP regions. The CIST is responsible for creating a loop-free topology by exchanging and propagating BPDUs across regions to form a Common Spanning Tree (CST).

MSTP uses RSTP as its converging algorithm and is interoperable with the legacy STP protocols: STP (802.1D) and RSTP (802.1w).

MSTP has three major advantages over 802.1D, 802.1w, and other proprietary implementations:
  • To save control path bandwidth and provide improved scalability, MSTP uses regions to localize BPDU traffic. BPDUs containing information about MSTIs contained within an MSTP region do not cross that region‘s boundary.

  • A single BPDU transmitted from a port can contain information for up to 64 STPDs. MSTP BPDU processing utilizes less resources compared to 802.1D or 802.1w where one BPDU corresponds to one STPD (Spanning Tree Domain).

  • In a typical network, a group of VLANs usually share the same physical topology. Dedicating a spanning tree per VLAN like PVST+ is CPU intensive and does not scale very well. MSTP makes it possible for a single STPD to handle multiple VLANs.