3Com 4200G 12-Port Configuration Manual page 130

116 C 20: MSTP C
HAPTER ONFIGURATION
In Figure 37, switch A, B, C, and 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. Figure 37
shows the roles these ports play.
A port can play different roles in different MSTIs.
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The role a region edge port plays is consistent with the role it plays in the CIST. For
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example, port 1 on switch A in Figure 37 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 37 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
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BPDU packets.
Learning state: Ports in this state can receive/send BPDU packets.
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Discarding state: Ports in this state can only receive BPDU packets.
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Table 83 lists possible combinations of port states and port roles.
Table 83 Combinations of port states and port roles
Port
state Forwarding
Learning
Discarding
Root/
port/Master Designated
port port
X X
X X
X X
Port role
Region
edge Alternate
port port
X —
X —
X X
Backup
port
—
—
X