1-12
z 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
z 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.
z 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.
z A port can play different roles in different MSTIs.
z 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
Edge ports
Connected to the
common port
Port 1 Port 2
Port 3 Port 4
Port 5
Port 6
MST
region
Designated
port
Master
port
Backup port
Alternate port
A
B
D
C
In MSTP, a port can be in one of the following three states: