MSTP key concepts

MSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculated CST. Inside an MST region, multiple spanning trees, called MSTIs, are calculated. Among these MSTIs, MSTI 0 is the internal spanning tree (IST).

The following diagram shows a switched network that comprises four MST regions, with each MST region comprising four MSTP devices.

The following diagram shows the networking topology of MST region 3.

MST region

A multiple spanning tree region (MST region) consists of multiple devices in a switched network and the network segments between them. All these devices have the following characteristics:

  • A spanning tree protocol enabled.
  • Same region name.
  • Same VLAN-to-instance mapping configuration.
  • Same MSTP revision level.
  • Physically linked together.

Multiple MST regions can exist in a switched network. You can assign multiple devices to the same MST region.

  • The switched network comprises four MST regions, MST region 1 through MST region 4.
  • All devices in each MST region have the same MST region configuration.


MSTP can generate multiple independent spanning trees in an MST region, and each spanning tree is mapped to specific VLANs. Each spanning tree is referred to as a multiple spanning tree instance (MSTI). In the diagrams, MST region 3 comprises three MSTIs, MSTI 1, MSTI 2, and MSTI 0.


VLAN-to-instance mapping table

As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping relationships between VLANs and MSTIs.

In the diagrams, the VLAN-to-instance mapping table of MST region 3 is as follows:
  • VLAN 1 to MSTI 1. (Ports which are not part of any VLAN are by default part of VLAN 1.)
  • VLAN 2 and VLAN 3 to MSTI 2.
  • Other VLANs to MSTI 0. (VLANs that are not configured as part of any MSTI are by default part of MSTI 0. )

MSTP achieves load balancing by means of the VLAN-to-instance mapping table.


The common spanning tree (CST) is a single spanning tree that connects all MST regions in a switched network. If you regard each MST region as a device, the CST is a spanning tree calculated by these devices through STP or RSTP. The blue lines in the diagrams represent the CST.


An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is also called MSTI 0, a special MSTI to which all VLANs are mapped by default. In the diagrams, MSTI 0 is the IST in MST region 3.


The common and internal spanning tree (CIST) is a single spanning tree that connects all devices in a switched network. It consists of the ISTs in all MST regions and the CST. In the diagrams, the ISTs (MSTI 0) in all MST regions plus the inter-region CST constitute the CIST of the entire network.

Regional root

The root bridge of the IST or an MSTI within an MST region is the regional root of the IST or MSTI. Based on the topology, different spanning trees in an MST region might have different regional roots, as shown in MST region 3 in the diagrams:

  • The regional root of MSTI 1 is Device B.
  • The regional root of MSTI 2 is Device C.
  • The regional root of MSTI 0 (also known as the IST) is Device A.

Common root bridge

The common root bridge is the root bridge of the CIST. In the diagrams, the common root bridge is a device in MST region 1.

Port roles

A port can play different roles in different MSTIs. In the following diagram, an MST region comprises Device A, Device B, Device C, and Device D. Port A1 and port A2 of Device A connect to the common root bridge. Port B2 and Port B3 of Device B form a loop. Port C3 and Port C4 of Device C connect to other MST regions. Port D3 of Device D directly connects to a host.

MSTP calculation involves the following port roles:

  • Root port: Forwards data for a non-root bridge to the root bridge. The root bridge does not have any root port.
  • Designated port: Forwards data to the downstream network segment or device.
  • Alternate port: Acts as the backup port for a root port or master port. When the root port or master port is blocked, the alternate port takes over.
  • Backup port: Acts as the backup port of a designated port. When the designated port is invalid, the backup port becomes the new designated port. A loop occurs when two ports of the same spanning tree device are connected, so the device blocks one of the ports. The blocked port acts as the backup.
  • Edge port: Does not connect to any network device or network segment, but directly connects to a user host.
  • Master port: Acts as a port on the shortest path from the local MST region to the common root bridge. The master port is not always located on the regional root. It is a root port on the IST or CIST and still a master port on the other MSTIs.
  • Boundary port: Connects an MST region to another MST region or to an STP/RSTP-running device. In MSTP calculation, a boundary port's role on an MSTI is consistent with its role on the CIST. However, that is not true with master ports. A master port on MSTIs is a root port on the CIST.

Port states

In MSTP, a port can be in one of the following states:

  • Forwarding: The port receives and sends BPDUs, learns MAC addresses, and forwards user traffic.
  • Learning: The port receives and sends BPDUs, learns MAC addresses, but does not forward user traffic. Learning is an intermediate port state.
  • Discarding: The port receives and sends BPDUs, but does not learn MAC addresses or forward user traffic.

When in different MSTIs, a port can be in different states.

A port state is not exclusively associated with a port role. Table 8 lists the port states that each port role supports. (A check mark [√] indicates that the port supports this state, while a dash [—] indicates that the port does not support this state.)

Port state Port role
Root port/master port Designated port Alternate port Backup port
Forwarding X X    
Learning X X    
Discarding X X X X

CIST calculation

During the CIST calculation, the following process takes place:

  • The device with the highest priority is elected as the root bridge of the CIST.
  • MSTP generates an IST within each MST region through calculation.
  • MSTP regards each MST region as a single device and generates a CST among these MST regions through calculation.

The CST and ISTs constitute the CIST of the entire network.

MSTI calculation

Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to-instance mappings. For each spanning tree, MSTP performs a separate calculation process similar to spanning tree calculation in STP.

In MSTP, a VLAN packet is forwarded along the following paths:

  • Within an MST region, the packet is forwarded along the corresponding MSTI.
  • Between two MST regions, the packet is forwarded along the CST.