3Com Switch 4800G 24-Port Configuration Manual page 209

The common root bridge is the root bridge of the CIST.
In Figure 56, for example, 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 calculation, a boundary port assumes the same role on the CIST and
on MST instances. Namely, if a boundary port is the master port on the CIST, it is
also the master port on all MST instances within this region. In Figure 56, for
example, 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.
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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 calculation 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.
A master port connects an MST region to the common root. The path from the
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master port to the common root is the shortest path between the MST region
and the common root. In the CST, the master port is the root port of the
region, which is considered as a node. The master port is a special boundary
port. It is a root port in the IST/CIST while a master port in the other MSTIs.
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 209