A port state is not exclusively associated with a port role.
where "√" indicates that the port supports the state and "—" indicates that the port does not support the
Ports states supported by different port roles
Port role (right)
Port state (below)
How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are interconnected by a
calculated CST. Inside an MST region, multiple spanning trees, called MSITs, are calculated. Among these
MSTIs, MSTI 0 is the CIST.
Similar to STP, MSTP uses configuration BPDUs to calculate spanning trees. However, an important
difference is that an MSTP BPDU carries the MSTP configuration on the device from which this BPDU is sent.
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, and, 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.
Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to-MSTI
mappings. For each spanning tree, MSTP performs a separate calculation process similar to spanning tree
calculation in STP. For more information, see
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.
Implementation of MSTP 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 following functions are provided for ease of management:
Root bridge hold
Root bridge backup
lists the port states supported by each port role,