3Com 4200G 12-Port Configuration Manual page 179

switch blocks one of the two ports to eliminate the loop that occurs. The blocked port is the backup
port.
In Figure 1-5, switch A, switch B, switch C, and switch D form an MST region. Port 1 and port 2 on
switch A connect upstream to the common root. Port 5 and port 6 on switch C form a loop. Port 3 and
port 4 on switch D connect downstream to other MST regions. This figure shows the roles these ports
play.
A port can play different roles in different MSTIs.
The role a region boundary port plays in an MSTI is consistent with the role it plays in the CIST. The
master port, which is a root port in the CIST while a master port in the other MSTIs, is an exception.
For example, in Figure
while a master port in all the other MSTIs in the region.
Figure 1-5 Port roles
Port state
In MSTP, a port can be in one of the following three states:
Forwarding state. Ports in this state can forward user packets and receive/send BPDU packets.
Learning state. Ports in this state can receive/send BPDU packets but do not forward user packets.
Discarding state. Ports in this state can only receive BPDU packets.
Port roles and port states are not mutually dependent.
states and port roles.
1-5, port 1 on switch A is a region boundary port. It is a root port in the CIST
1-12
Table 1-6 lists possible combinations of port