MSTP Topology Calculation

MSTP can divide the entire Layer 2 network into multiple MST regions. The CST is generated through calculation. 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. These configuration messages, however, are MSTP-specific.

Vectors

Both MSTIs and the CIST are calculated based on vectors, carried in MST BPDUs.

There are seven types of vectors used to calculate MSTIs and the CIST. Each vector carries a priority value. For each vector, smaller priority values indicate higher priorities.

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 message and 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 that saved on the port, the port discards the BPDU. Table 1 describes each vector.

**Table 1** Vector description
Vector Name	Description
Root ID	Identifies the root switch 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 are the same on all switches in an MST region. If the CIST root is in an MST region, all ERPCs in that MST region are set to 0.
Regional root ID	Identifies the MSTI regional root and 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 must be 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 same as the local bridge ID.
Designated port ID	Identifies the port on the designated switch connected to the root port on the local bridge. The designated 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 receiving port ID consists of the priority value (4 bits) and port number (12 bits). The priority value must be a multiple of 16.

The following vectors are used in CIST calculation:

Root ID
External root path cost
Region root ID
Internal root path cost
Designated switch ID
Designated port ID
Receiving port ID

The following vectors are used in MSTI calculation:

Regional root ID
Internal root path cost
Designated switch ID
Designated port ID
Receiving port ID

The preceding vectors are listed in descending order of priority.

Vectors are compared in the following sequence:

Root IDs
ERPCs
Regional root IDs
IRPCs
Designated switch IDs
Designated port IDs
Receiving port IDs

If the vectors being compared are the same, the next vector in the list is compared. If the vectors being compared are different, the remaining vectors are not compared

CIST Calculation

After comparing the vectors, the switch with the highest priority on the entire network is selected as the CIST root. MSTP calculates an IST for each MST region, and calculates a CST to interconnect MST regions. The CST and ISTs form a CIST for the entire network.

MSTI Calculation

In an MST region, MSTP independently calculates an MSTI for each VLAN based on mappings between VLANs and MSTIs. The calculation process is similar to that used by 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. Spanning trees of MSTIs are independent of each other.
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 a similar manner to how RSTP topology changes are processed. For details, see Improvements Made in RSTP.