3Com MSR 50 Series Configuration Manual page 455

The root bridge of the IST or an MSTI within an MST region is the regional root
bridge of the MST or that MSTI. Based on the topology, different spanning trees in
an MST region may have different regional roots. For example, in region D0 in
Figure 130, the regional root of instance 1 is device B, while that of instance 2 is
device C.
8 Common root bridge
The common root bridge is the root bridge of the CIST. In
the common root bridge is a device in region A0.
9 Boundary port
A boundary port is a port that connects an MST region to another MST
configuration, or to a single spanning-tree region running STP, or to a single
spanning-tree region running RSTP.
During MSTP computing, a boundary port assumes the same role on the CIST and
on MST instances. Namely, if a boundary port is master port on the CIST, it is also
the master port on all MST instances within this region. In
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 device is not capable of recognizing boundary ports. When the
device interworks with 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 root port, designated port,
master port, alternate port, backup port, and so on.
Root port: a port responsible for forwarding data to the root bridge.
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Designated port: a port responsible for forwarding data to the downstream
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network segment or device.
Master port: A port on the shortest path from the entire region to the common
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root bridge, connecting the MST region to the common root bridge.
Alternate port: The standby port for the root port or master port. When the
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root port or master port is blocked, the alternate port becomes the new root
port or master port.
Backup port: The backup port of designated ports. When a designated port is
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blocked, the backup port becomes a new designated port and starts
forwarding data without delay. When a loop occurs while two ports of the
same MSTP 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.
MSTP Overview 455
Figure 130, for example,
Figure 130, for example,