MSTP Topology Calculation

MSTP Principle

In Multiple Spanning Tree Protocol (MSTP), the entire Layer 2 network is divided into multiple MST regions, which are interconnected by a single Common Spanning Tree (CST). In a Multiple Spanning Tree (MST) region, multiple spanning trees are calculated, each of which is called a Multiple Spanning Tree Instances (MSTI). Among these MSTIs, MSTI 0 is also known as the internal spanning tree (IST). Like STP, MSTP uses configuration messages to calculate spanning trees, but the configuration messages are MSTP-specific.

Vectors

Both MSTIs and the CIST are calculated based on vectors, which are carried in Multiple Spanning Tree Bridge Protocol Data Units (MST BPDUs). Therefore, switching devices exchange MST BPDUs to calculate MSTIs and the Common and Internal Spanning Tree (CIST).

• Vectors are described as follows:

  • The following vectors participate in the CIST calculation:

    {root ID, external root path cost, region root ID, internal root path cost, designated switching device ID, designated port ID, receiving port ID}

  • The following vectors participate in the MSTI calculation:

    {regional root ID, internal root path cost, designated switching device ID, designated port ID, receiving port ID}

  The priorities of vectors in braces are in descending order from left to right.

  Table 1 describes the vectors.

  Table 1 Vector description

  Vector Name	Description
  Root ID	Identifies the root switching device for the CIST. The root identifier consists of the priority value (16 bits) and MAC address (48 bits).
  External root path cost (ERPC)	Indicates the path cost from a CIST regional root to the root. ERPCs saved on all switching devices in an MST region are the same. If the CIST root is in an MST region, ERPCs saved on all switching devices in the MST region are 0s.
  Regional root ID	Identifies the MSTI regional root. The regional root ID consists of the priority value (16 bits) and MAC address (48 bits).
  Internal root path cost (IRPC)	Indicates the path cost from the local bridge to the regional root. The IRPC saved on a regional edge port is greater than the IRPC saved on a non-regional edge port.
  Designated switching device ID	Identifies the nearest upstream bridge on the path from the local bridge to the regional root. If the local bridge is the root or the regional root, this ID is the local bridge ID.
  Designated port ID	Identifies the port on the designated switching device connected to the root port on the local bridge. The port ID consists of the priority value (4 bits) and port number (12 bits). The priority value must be a multiple of 16.
  Receiving port ID	Identifies the port receiving the BPDU. The port ID consists of the priority value (4 bits) and port number (12 bits). The priority value must be a multiple of 16.

• The vector comparison principle is as follows:

  For a vector, the smaller the priority value, the higher the priority.

  Vectors are compared based on the following rules:

  1. Compare the IDs of the roots.

  2. If the IDs of the roots are the same, compare ERPCs.

  3. If ERPCs are the same, compare the IDs of regional roots.

  4. If the IDs of regional roots are the same, compare IRPCs.

  5. If IRPCs are the same, compare the IDs of designated switching devices.

  6. If the IDs of designated switching devices are the same, compare the IDs of designated ports.

  7. If the IDs of designated ports are the same, compare the IDs of receiving ports.

  If the priority of a vector carried in the configuration message of a BPDU received by a port is higher than the priority of the vector in the configuration message saved on the port, the port replaces the saved configuration message with the received one. In addition, the port updates the global configuration message saved on the device. If the priority of a vector carried in the configuration message of a BPDU received on a port is equal to or lower than the priority of the vector in the configuration message saved on the port, the port discards the BPDU.

CIST Calculation

After completing the configuration message comparison, the switching device with the highest priority on the entire network is selected as the CIST root. MSTP calculates an IST for each MST region, and computes a CST to interconnect MST regions. On the CST, each MST region is considered a switching device. The CST and ISTs constitute a CIST for the entire network.

MSTI Calculation

In an MST region, MSTP calculates an MSTI for each VLAN based on mappings between VLANs and MSTIs. Each MSTI is calculated independently. The calculation process is similar to the process for STP to calculate a spanning tree. For details, see STP Topology Calculation.

MSTP Responding to Topology Changes

MSTP topology changes are processed in the manner similar to that in RSTP. For details about how RSTP processes topology changes, see RSTP Implementation.