3Com Switch 7750 Configuration Guide

244 CHAPTER 30: MSTP CONFIGURATION
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.
n
■ 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
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
Root/
port/Maste
r port
Designated
port
Region
edge port
Alternate
port
Backup
port
Forwarding √√ √ - -
Learning √ √ √ - -
Discarding √ √ √ √ √
A
B
D
C
Connecting to the
common root bridge
Edge ports
MST region
Port 1
Master port
Alternate port
Port 6
Backup port
Designated
port
Port 4
Port 5
Port 3
Port 2