port state is available for the corresponding port role, and a dash [—] indicates that the port state is not available for the corresponding port role.)

Table 57 Ports states supported by different port roles

 

Port role

 

 

 

 

Port state

Root port/master

Designated

Boundary

Alternate port

Backup port

 

port

port

port

 

 

 

Forwarding

√

√

√

—

—

 

 

 

 

 

 

Learning

√

√

√

—

—

 

 

 

 

 

 

Discarding

√

√

√

√

√

 

 

 

 

 

 

How MSTP works

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 CIST.

Similar to RSTP, MSTP uses configuration BPDUs to calculate spanning trees. An important difference is that an MSTP BPDU carries the MSTP configuration of the bridge from which the BPDU is sent.

CIST calculation

The calculation of a CIST tree is also the process of configuration BPDU comparison. During this process, 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. At the same time, 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, which is similar to spanning tree calculation in STP/RSTP. For more information, see "Calculation process of the STP algorithm"

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.

MSTP implementation on devices

MSTP is compatible with STP and RSTP. STP and RSTP protocol packets can be recognized by devices running MSTP and used for spanning tree calculation.

In addition to basic MSTP functions, the device provides the following functions for ease of management:

•Root bridge hold

•Root bridge backup

•Root guard

•BPDU guard

189