3Com WX3000 operation manual Port state, MSTP, a port can be in one of the following three states

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A region edge port is located on the edge of an MST region and is used to connect one MST region to another MST region, an STP-enabled region or an RSTP-enabled region

An alternate port is a secondary port of a root port or master port and is used for rapid transition. With the root port or master port being blocked, the alternate port becomes the new root port or master port.

A backup port is the secondary port of a designated port and is used for rapid transition. With the designated port being blocked, the backup port becomes the new designated port fast and begins to forward data seamlessly. When two ports of an MSTP-enabled device are interconnected, the device blocks one of the two ports to eliminate the loop that occurs. The blocked port is the backup port.

In Figure 1-5, device A, device B, device C, and device D form an MST region. Port 1 and port 2 on device A connect upstream to the common root. Port 5 and port 6 on device C form a loop. Port 3 and port 4 on device D connect downstream to other MST regions. This figure shows the roles these ports play.

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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 device A in Figure 1-5 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 1-5 Port roles

Connected to the

Edge ports common port

MST	Port 1
region

Master

port

B

Designated

port Port 3

Port 2

A

Alternate port

C

Port 6

Port 5

D

Backup port

Port 4

Port state

In MSTP, a port can be in one of the following three states:

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