1-12
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.
z A port can play different roles in different MSTIs.
z 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.
z For example, in Figure 1-5, port 1 on switch A is a region boundary port. It is a root port in the CIST
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:
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 but do not forward user 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.
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