3Com Switch 8800 Advanced Software V5 Configuration Guide

MSTP Overview 105
if a device in region A0 is interconnected with the first port of a device in region
D0 and the common root bridge of the entire switched network is located in
region A0, the first port of that device in region D0 is the boundary port of region
D0.
n
Currently, the Switch 8800s are not capable of recognizing boundary ports. When
a Switch 8800 is connected to a third party’s device that supports boundary port
recognition, the third party’s device may malfunction in recognizing a boundary
port.
10 Roles of ports
In the MSTP computing process, port roles include designated port, root port,
master port, alternate port, backup port, and so on.
■ Root port: a port responsible for forwarding data to the root bridge.
■ Designated port: a port responsible for forwarding data to the downstream
network segment or device.
■ Master port: A port on the shortest path from the entire region to the common
root bridge, connect the MST region to the common root bridge.
■ Alternate port: The standby port for a master port. If a master port is blocked,
the alternate port becomes the new master port.
■ Backup port: If a loop occurs when two ports of the same device are
interconnected, the device will block either of the two ports, and the backup
port is that port to be blocked.
A port can assume different roles in different MST instances.
Figure 29 Port roles
In Figure 29,
■ Devices A, B, C, and D constitute an MST region.
■ Port 1 and port 2 of device A connect to the common root bridge.
■ Port 5 and port 6 of device C form a loop.
Connecting to the
common root bridge
Edge port
Port 1
2
Master port Alternate port
Designated
port
34
5
A
B
C
D
6
Backup port
MST region
Port
Port
Port
PortPort