3Com 7757 Configuration Manual page 244

244 C 30: MSTP C
HAPTER
n
ONFIGURATION
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 edge port plays is consistent with the role it plays in the CIST.
■
For example, port 1 on switch A in Figure 61 is a region edge port, and it is a
master port in the CIST. So it is a master port in all MSTIs in the region.
Figure 61 Port roles
Edge ports
MST region
Master port
Designated
port
Port states
Ports can be in 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.
■
Discarding state: Ports in this state can only receive BPDU packets.
■
Table 170 lists possible combinations of port states and port roles.
Table 170 Combinations of port states and port roles
Port State \ Port
Role
Forwarding
Learning
Discarding
Connecting to the
common root bridge
Port 1 Port 2
A
B
D
Port 4
Port 3
Root/
port/Maste Designated
r port port
√ √
√ √
√ √
Alternate port
C
Port 6
Port 5
Backup port
Region Alternate
edge port port
√
-
√
-
√ √
Backup
port
-
-
√