Multiple Spanning Tree Protocol (MST)

Multiple Spanning Tree (MST) extends the IEEE 802.1w RST algorithm to multiple spanning trees. The main purpose of MST is to reduce the  total number of spanning-tree instances to match the physical topology of the network and thus reduce the CPU cycles of a switch. MST converges faster than PVRST+ and is backward compatible with 802.1D STP, 802.1w (RSTP), and the Cisco PVSTP+ architecture.

MST allows for the building of multiple spanning trees over trunks by grouping and associating VLANs to spanning-tree instances. Each instance may have a topology that is independent of other spanning-tree instances. This architecture provides multiple forwarding paths for data traffic and enables load balancing. A failure in one forwarding path does not affect other instances with different forwarding paths; hence, this architecture improves network fault tolerance. Configuring a set of bridges with the same MST configuration information allows them to participate in a specific set of spanning-tree instances. The term MST region refers to the set of interconnected bridges that have the same MST configuration.

When running MST within your network you can achieve the following results;

  • Create load-balancing scheme by putting half of your VLANs in one MST instance and your other half into a second instance.
  • The switch utilization is low because it has to only handle two instances.

MST enables you to build multiple spanning trees over trunks by grouping VLANs and  associating them with spanning-tree instances. Each instance can have a topology independent of other spanning-tree instances. This architecture provides multiple active forwarding paths for data traffic and enables load balancing.

Fault tolerance in a network is improved compared to CST. If a failure in one instance (FWD path) does not necessarily affect other instances. This type of grouping, VLAN-to-MST, must be consistent across all bridges within an MST region. You must configure a set of bridges with the same MST configuration information, which allows them to participate in a specific set of spanning-tree instances. Interconnected bridges that have the same MST configuration are referred to as an MST region. Bridges with different MST configurations or legacy bridges running 802.1D are considered separate MST regions.

MST Regions: An MST Region defines a boundary within which a single instance of STP  operates. MST employs the use of Regions because not all switches in the network might run or support MST; therefore, different kinds of STPs divide the network into STP regions. In the case of the 802.1Q standard, all instances map to a unique and common instance and are therefore less complex. In the case of PVST+, each VLAN carries the BPDUs for its respective instance
(one BPDU per VLAN).

A collection of interconnected switches that have the same MST configuration comprises an MST Region. MST configuration must be the same on all switches in the same MST region; this includes the following three user-configured parameters:

  1. The MST Region Name (up to 32 bytes)
  2. Configuration Revision Number (0 to 65,535)
  3. VLAN-To-Instance Mapping (up to 4,096 entries)

In order for two or more switches to be in the same MST Region, they must have the same VLAN-To-Instance Mapping, Configuration Revision Number, and MST Region Name. If not, the switches belong to two independent Regions. To ensure a consistent VLANs-to-instance  mapping, the protocol must exactly identify the boundaries of the regions. For that purpose, the characteristics of the region are included in BPDUs. Switches do not propagate the exact VLANs-to-instance mapping in the BPDU because the switches need to know only whether they are in the same region as a neighbor. Switches will only send a digest of the VLANsto-instance mapping table, along with the revision number and the name. If the digest that are received within the MST BPDU do not match that of the receiving switch, the port that received the BPDU is seen as been at boundary of the region.

MST Region Components: An MST Region consists of two different components,

  1. Edge port – a port that connects to a non-bridging device, such as a network host. Additionally, a port that connects to a hub is also considered an edge port.
  2. Boundary port – connects an MST Region to a single STP Region running RSTP, 802.1D, or to another MST Region with a different MST configuration. Also an MST boundary port may also connect to a LAN, or to the Designated Switch that belongs to a single STP instance or another MST instance.


MST Spanning-tree Instances: Unlike PVST+, in which all STP instances are independent, or CST, where only a single STP instance exists, MST establishes and maintains the two types of STP instances;

  1. Internal Spanning Tree (IST)
  2. Multiple Instance Spanning Tree Protocol (MISTP)

Internal Spanning Tree (IST): The MST Region sees the outside world via its Internal Spanning Tree (IST) interaction only, the IST will present the MST Region as one single virtual bridge to the outside world. This leads to only BPDU been sheared over the native VLAN of the trunks, as it would do if it where running in CST mode. IST is the only Instance that sends and receives BPDUs. Other STP instances which are running are contained in the M-record which is encapsulated within the BPDU. Because the information for multiple instances can be found within a BPDU, this helps to reduces the processing a switch needs to do.

This is how the outside world will see a MST Region been presented to them;


At the boundary of the MST Region, the Root Path Cost and Message Age values are incremented as though the BPDU had traversed only a single switch.

IST Master: The IST connects all the MST switches within a Region. When the IST converges, the Root of the IST becomes the IST master. IST master election takes the same process as STP Root Bridge election, lowest BID wins the election. Switches within the Region then exchange BPDUs and eventually the switch within the region with the lowest BID and Path Cost to the CST Root is elected IST master. If there is only a single Region, then the IST master will also be the CST Root. However, if the CST Root is outside of the Region, one of the MST switches at the boundary of the Region is selected as the IST master. In such cases, at the boundary, the MST switch adds the IST master ID as well as the IST master Path Cost to the BPDU, which is then propagated throughout the MST Region.


Hop Count: IST and MIST do not use the Message Age and Maximum Age information in the configuration BPDU to compute STP topology. Instead, they use the Path Cost to the Root and a hop-count mechanism, which is similar to the IP TTL mechanism. The hop-count mechanism give us the same results as Message Age information would in determining when to trigger a reconfiguration. The Root Bridge of the Instance always sends a BPDU (or M-record) with a cost of 0 and the hop count set to the maximum value (20 by default). When a switch receives this BPDU is removes one of the hop counts before sending it on to the next downstream switch. When the count reaches zero, the switch discards the BPDU and ages the information held for the port. The Message Age and Maximum Age information in the BPDU remain the same throughout the Region, also these same values are propagated by the Region’s Designated Ports at the boundary.

Multiple Instance Spanning Tree Protocol (MIST): Multiple Instance Spanning Tree Protocol (MISTP) represents STP instances that exist only within the region. The IST presents the entire region as a single virtual bridge to the CST outside.MISTP however does not interact directly with the CST outside of the region.

MISTP 0 is mandatory and is always present; however, all other Instances are optional. Each MISTP typically maps to a VLAN or set of VLANs. By default, all VLANs are assigned to the IST, which is MISTP 0. Cisco Catalyst switches support up to 16 MISTPs on a single switch. These are identified by the numbers 0 through 15.

– IST exists on all ports within the MST Region, and only the IST has timer-related parameters. Also unlike in traditional Spanning Tree Protocol, MST BPDUs are sent out of every port by all switches, versus being sent out only by the Designated Bridge. However, that MISTP does not send BPDUs out of boundary ports because only the IST interacts with external STP.