Spanning Tree Protocol (STP) is a vital protocol used in Ethernet networks to prevent loops and ensure a loop-free topology. MST (Multiple Spanning Tree) protocol is an extension of STP that provides greater flexibility and efficiency in managing redundant paths. In this article, we will delve into MST, its features, commands, and provide examples of its configuration. We will also highlight some lesser-known aspects of MST that are not commonly found in other articles.

Introduction to MST:

MST is defined in IEEE 802.1s and offers significant improvements over the original STP. MST allows for the creation of multiple spanning tree instances, each representing a specific group of VLANs. This feature enables network administrators to optimize the utilization of redundant paths based on VLAN groupings.

Key Features of MST:

  1. Multiple Spanning Tree Instances: MST allows the creation of multiple instances, known as MST instances or regions. Each MST instance can have a group of VLANs associated with it, and the network topology is computed separately for each instance. This provides better control and load balancing in complex network environments.
  2. Instance Mapping: MST maps VLANs to specific MST instances based on their VLAN IDs. This mapping is configured on each switch and ensures that VLANs are assigned to the correct MST instance. By carefully grouping VLANs, network administrators can optimize the path selection and avoid unnecessary reconvergence.
  3. Shared Spanning Tree: MST instances that share the same configuration parameters, including VLAN-to-instance mappings, are grouped together into a region. Within each region, a single common spanning tree, called the Common and Internal Spanning Tree (CIST), is used to maintain connectivity and handle traffic between instances.
  4. Root Bridge Election: MST implements a per-instance root bridge election process. This means that different MST instances can have different root bridges, allowing for independent topology computation and load balancing based on VLAN groups.
  5. Rapid Spanning Tree Convergence: MST incorporates Rapid Spanning Tree Protocol (RSTP) enhancements, such as port roles (root, designated, alternate, and backup) and port states (forwarding, learning, and blocking), to achieve faster convergence and reduce network downtime.

Configuration of MST:

The configuration of MST involves the following steps:

  1. Configure the MST region name: This step assigns a name to the MST region for identification purposes. Use the following command:

Switch(config)# spanning-tree mst configuration
Switch(config-mst)# name <region-name>

  1. Configure the VLAN-to-instance mapping: Specify which VLANs belong to each MST instance. Use the following command:

Switch(config)# spanning-tree mst configuration
Switch(config-mst)# instance <instance-id> vlan <vlan-range>

Replace <instance-id> with the MST instance identifier, and <vlan-range> with the range of VLANs assigned to that instance.

  1. Set the priority of the switches: Assign priorities to switches to influence the root bridge selection process. Use the following command:

Switch(config)# spanning-tree mst configuration
Switch(config-mst)# instance <instance-id> priority <priority-value>

Replace <instance-id> with the MST instance identifier, and <priority-value> with a numerical value indicating the switch priority.

  1. Enable MST on the switch: Activate MST on the switch using the following command:

Switch(config)# spanning-tree mode mst

Rarely Known Aspects of MST:

  1. MST Region Revision Number: Each MST region has a revision number associated with it. This number increases whenever a change is made to the MST configuration. The revision number is used for synchronization purposes and to ensure consistent MST configuration among switches in the region.
  2. MST Digests: MST uses a digest algorithm to compare MST configurations between switches in the same region. This ensures that the switches have compatible MST configurations before forming a valid MST region.
  3. Migration from PVST+ to MST: Cisco devices support a feature called Multiple Spanning Tree (MST) Conversion Mode, which allows seamless migration from Per-VLAN Spanning Tree (PVST+) or Rapid PVST+ to MST without causing network disruption.

Example Configuration:

Let’s consider an example where we configure MST on a network with two switches, SwitchA and SwitchB, belonging to the same MST region. We assign VLANs 10-20 to MST instance 1 and VLANs 30-40 to MST instance 2.

SwitchA configuration:

SwitchA(config)# spanning-tree mst configuration
SwitchA(config-mst)# name MST_REGION
SwitchA(config-mst)# instance 1 vlan 10-20
SwitchA(config-mst)# instance 2 vlan 30-40
SwitchA(config-mst)# instance 1 priority 4096
SwitchA(config-mst)# instance 2 priority 8192
SwitchA(config)# spanning-tree mode mst

SwitchB configuration:

SwitchB(config)# spanning-tree mst configuration
SwitchB(config-mst)# name MST_REGION
SwitchB(config-mst)# instance 1 vlan 10-20
SwitchB(config-mst)# instance 2 vlan 30-40
SwitchB(config-mst)# instance 1 priority 8192
SwitchB(config-mst)# instance 2 priority 4096
SwitchB(config)# spanning-tree mode mst

In this example, both switches are part of the same MST region, with different priorities for each MST instance. VLANs 10-20 will be calculated by MST instance 1, while VLANs 30-40 will be handled by MST instance 2.


MST (Multiple Spanning Tree) protocol is a powerful enhancement to Spanning Tree Protocol (STP), providing flexibility and efficiency in managing redundant paths in complex networks. By understanding the features, configuration steps, and lesser-known aspects of MST, network administrators can optimize network performance, improve convergence times, and achieve better load balancing. With its ability to create multiple spanning tree instances, MST is a valuable protocol for modern network infrastructures.