1-13
z Forwarding state. Ports in this state can forward user packets and receive/send BPDU packets.
z Learning state. Ports in this state can receive/send BPDU packets.
z Discarding state. Ports in this state can only receive BPDU packets.
Port roles and port states are not mutually dependent. Table 1-6 lists possible combinations of port
states and port roles.
Table 1-6 Combinations of port states and port roles
Port role
(right)
Port state
(below)
Root/
port/Master
port
Designated
port Region edge
port Alternate
port Backup port
Forwarding √ √ √ — —
Learning √ √ √ — —
Discarding √ √ √ √ √
MSTP divides a Layer 2 network into multiple MST regions. The CSTs are generated between these
MST regions, and multiple spanning trees (also called MSTIs) can be generated in each MST region. As
well as RSTP, MSTP uses configuration BPDUs for spanning tree calculation. The only difference is that
the configuration BPDUs for MSTP carry the MSTP configuration information of the devices.
Calculate the CIST
Through comparing configuration BPDUs, the device with the highest priority in the network is selected
as the root of the CIST. In each MST region, an IST is calculated by MSTP. At the same time, MSTP
regards each MST region as a device to calculate the CSTs of the network. The CSTs, together with the
ISTs, form the CIST of the network.
Calculate an MSTI
In an MST region, different MSTIs are generated for different VLANs based on the VLAN-to-MSTI
mappings. Each spanning tree is calculated independently, in the same way as how STP/RSTP is
calculated.
Implement STP algorithm
In the beginning, each device regards itself as the root, and generates a configuration BPDU for each
port on it as a root, with the root path cost being 0, the ID of the designated bridge being that of the
device, and the designated port being itself.
1) Each device sends out its configuration BPDUs and operates in the following way when receiving a
configuration BPDU on one of its ports from another device:
z If the priority of the configuration BPDU is lower than that of the configuration BPDU of the port
itself, the device discards the BPDU and does not change the configuration BPDU of the port.
z If the priority of the configuration BPDU is higher than that of the configuration BPDU of the port
itself, the device replaces the configuration BPDU of the port with the received one and compares it
with those of other ports on the device to obtain the one with the highest priority.
2) Configuration BPDUs are compared as follows: