What is MSTP? | Multiple Spanning Tree Protocol

In modern network infrastructures, spanning tree protocols play a vital role in ensuring the stability and redundancy of networks. One such protocol is the Multiple Spanning Tree Protocol (MSTP), which provides a scalable and efficient solution for managing network loops.


What is MSTP?

MSTP, or Multiple Spanning Tree Protocol, is a crucial networking technology that plays a pivotal role in ensuring the stability and efficiency of modern networks. MSTP, also known as IEEE 802.1s, is an extension of the Rapid Spanning Tree Protocol (RSTP).

It allows for the creation of multiple spanning trees across a network, providing better load balancing and faster convergence times compared to traditional Spanning Tree Protocol (STP).MSTP accomplishes this by creating multiple spanning tree instances within a network, each responsible for a specific subset of switches and ports.

Key Features of MSTP

  • Redundancy: MSTP enhances network reliability by allowing for redundant paths while avoiding loops.
  • Load Balancing: It optimizes network traffic distribution, preventing bottlenecks.
  • Efficiency: MSTP reduces the convergence time in case of network failures, ensuring minimal downtime.

How Does MSTP Work?

MSTP works by dividing the network into multiple regions, known as Multiple Spanning Tree Instances (MSTIs). Each MSTI can have its own spanning tree topology, allowing for greater flexibility in managing network traffic and optimizing network resources.

When a switch receives a BPDU (Bridge Protocol Data Unit) from another switch, it examines the MSTP configuration information in the BPDU to determine the appropriate MSTI to forward the traffic. This allows switches in different regions to have different spanning tree topologies, reducing the overall network complexity.

MSTP operates by dividing the network into multiple regions, each with its own spanning-tree instance. This division is based on the VLANs present in the network. Here’s a simplified overview of how MSTP works:

  1. Region Configuration: Network administrators configure MSTP regions, defining which switches belong to each region.
  2. Instance Creation: Within each region, MSTP creates a separate spanning tree instance.
  3. Bridge Priority: Each switch within a region is assigned a bridge priority, determining its role in the spanning tree.
  4. BPDU Exchange: Switches exchange Bridge Protocol Data Units (BPDUs) to elect the root bridge for each instance.
  5. Path Selection: MSTP calculates the shortest path to the root bridge for each instance, creating a loop-free topology.
  6. Port States: Ports transition through states like blocking, listening, learning, and forwarding to establish a stable network.

What is Multiple Spanning Tree Instances (MSTIs)

Multiple Spanning Tree Instances (MSTIs) is a crucial concept within the Multiple Spanning Tree Protocol (MSTP), a network protocol designed to prevent loops and enhance the reliability and efficiency of Ethernet networks. MSTIs are one of the key features that set MSTP apart from other spanning tree protocols like RSTP (Rapid Spanning Tree Protocol) and STP (Spanning Tree Protocol).

In simple terms, MSTIs allow network administrators to divide a network into multiple logical segments or instances, each with its own independent spanning tree. Each MSTI operates as if it were a separate spanning tree protocol instance, even though they share a common physical network infrastructure. Here’s a breakdown of the essential aspects of Multiple Spanning Tree Instances:

MSTIs enable the segmentation of a network into distinct regions, each with its own MSTP configuration. This segmentation is often based on VLANs (Virtual LANs), allowing network administrators to manage redundancy and topology control independently for different VLANs. Within each MSTI, a separate spanning tree is calculated. This means that the topology and path selection for one MSTI do not affect the others. It provides granular control over redundancy and network traffic for specific VLANs or network segments.

In MSTP, each switch within an MSTI is assigned a bridge priority value. The bridge with the lowest priority becomes the root bridge for that instance. This allows for flexibility in root bridge selection for different segments of the network.


What is BPDU (Bridge Protocol Data Unit)?

A Bridge Protocol Data Unit (BPDU) is a crucial element in computer networking, particularly in the context of spanning tree protocols like MSTP, RSTP, and STP. Network switches use BPDUs, or Bridge Protocol Data Units, to ensure a broadcast storm-free topology and avoid performance drops due to network loops.

Switches in a spanning tree network use the data contained in BPDU packets to figure out the best route to the root bridge. The topology of the network and the bridge’s identity and priority are among the details they contain.

Here are some key components and functions of Bridge Protocol Data Units (BPDUs):

  1. Bridge Identifier: BPDUs carry information about the sender bridge, including its unique Bridge ID and MAC address. This information helps switches identify each other and establish hierarchies within the spanning tree.
  2. Path Cost: BPDUs include the cost or distance from the sender to the root bridge. This cost is calculated based on various factors, such as link speed and bandwidth. It allows switches to determine the best path to the root bridge.
  3. Message Type: BPDUs can convey different types of messages, such as Configuration BPDUs and Topology Change Notification (TCN) BPDUs. Configuration BPDUs help in constructing the initial topology, while TCN BPDUs inform switches of network changes.
  4. Priority and Port Information: BPDUs also contain data about the priority of the sending switch and the port on which the BPDU was transmitted. This information assists switches in electing the root bridge and designating root ports.
  5. Hello Timers: BPDUs are sent at regular intervals as “hello” messages to ensure the network’s stability. If a switch stops receiving BPDUs from a specific port, it can infer that a link or switch has failed.

By facilitating communication between switches, determining the optimal network topology, and responding to network changes in real-time, BPDUs are crucial to the success of spanning tree protocols, which are responsible for keeping networks free of loops and functioning reliably.


MSTP offers several advantages over other spanning tree protocols like RSTP and STP. Firstly, MSTP provides better load balancing by allowing for the creation of multiple spanning trees. This means that network traffic can be distributed across different paths, improving overall network performance.

Secondly, MSTP offers faster convergence times compared to STP. With MSTP, only the affected MSTI needs to reconverge, while the rest of the network remains unaffected. This reduces the time taken for the network to recover from failures or changes in the network topology.

Lastly, MSTP provides greater flexibility in managing network resources. By dividing the network into multiple regions, network administrators can assign different VLANs to different MSTIs, allowing for more efficient use of network resources.

Full NameMultiple Spanning Tree ProtocolRapid Spanning Tree ProtocolSpanning Tree Protocol
IEEE Standard802.1s802.1w802.1D
Convergence TimeFaster convergenceRapid convergenceSlow convergence
VLAN SupportSupports multiple VLANsLimited VLAN supportLimited VLAN support
ComplexityMore complex configurationSimplified configurationBasic configuration
ScalabilityHighly scalableSuitable for small to medium-sizedSuitable for small networks
Configuration GranularityGranular control over VLANsLess granular controlBasic control
CompatibilityCompatibility may vary amongWidely supportedWidely supported
different vendors
Resource UtilizationEfficient resource utilizationEfficient resource utilizationLess efficient resource usage
Primary Use CasesLarge and complex networksSmaller networks with less complexitySimple networks

Advantages of MSTP

MSTP offers several advantages for network administrators and organizations:

  1. Enhanced Redundancy: Redundant paths ensure network resilience and failover capabilities.
  2. Efficient Resource Utilization: Load balancing optimizes network performance.
  3. Support for Multiple VLANs: MSTP is VLAN-aware, making it ideal for diverse networks.
  4. Scalability: Suited for networks of all sizes, from small offices to large enterprises.
  5. Reduced Downtime: Faster convergence minimizes network downtime.

Disadvantages of MSTP

While MSTP is a powerful protocol, it’s essential to consider its limitations:

  1. Complex Configuration: MSTP configuration can be intricate, requiring a deep understanding.
  2. Compatibility Issues: Not all switches support MSTP, which can lead to compatibility challenges.
  3. Resource Intensive: In large networks, MSTP can consume significant processing power.

MSTP is a powerful spanning tree protocol that offers better load balancing, faster convergence times, and greater flexibility in managing network resources. While it may introduce some complexity in network configuration and require more processing power and memory in switches, the benefits it provides make it a preferred choice in modern network environments. Understanding how MSTP works and its advantages and disadvantages is essential for network administrators to design and maintain stable and efficient networks.

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