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# Understanding of MSTP - MSTP Topology Calculation

Latest reply: May 22, 2021 08:30:01 292 38 21 0 0

OBJECTIVE

The purpose of this post is to present an introduction to the MSTP Topology Calculation.

MSTP Principle

In MSTP, the entire Layer 2 network is divided into multiple MST regions, which are interconnected by a single CST. In an MST region, multiple spanning trees are calculated, each of which is called an 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 MST BPDUs. Therefore, switching devices exchange MST BPDUs to calculate MSTIs and the 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.

 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).The priority value is the priority of MSTI 0. 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).The priority value is the priority of MSTI 0. 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.

Table 1 - Vector description

• 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.

MSTIs have the following characteristics:

• The spanning tree is calculated independently for each MSTI, and spanning trees of MSTIs are independent of each other.

• MSTP calculates the spanning tree for an MSTI in the manner similar to STP.

• Spanning trees of MSTIs can have different roots and topologies.

• Each MSTI sends BPDUs in its spanning tree.

• The topology of each MSTI is configured by using commands.

• A port can be configured with different parameters for different MSTIs.

• A port can play different roles or have different status in different MSTIs.

On an MSTP-aware network, a VLAN packet is forwarded along the following paths:

• MSTI in an MST region

• CST among MST regions

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 Details About RSTP.

--- End

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