Common root bridge
The common root bridge is the root bridge of the CIST.
In
, for example, the common root bridge is a device in MST region 1.
Roles of ports
A port can play different roles in different MSTIs. As shown in
, an MST region comprises Device
A, Device B, Device C, and Device D. Port A1 and port A2 of Device A connect to the common root
bridge. Port B2 and Port B3 of Device B form a loop. Port C3 and Port C4 of Device C connect to other
MST regions. Port D3 of Device D directly connects to a host.
Figure 16
Port roles
MSTP calculation involves the following port roles:
•
Root port: Forwards data for a non-root bridge to the root bridge. The root bridge does not have any
root port.
•
Designated port: Forwards data to the downstream network segment or device.
•
Alternate port: The backup port for a root port or master port. When the root port or master port is
blocked, the alternate port takes over.
•
Backup port: The backup port of a designated port. When the designated port fails, the backup
port takes over. When a loop occurs because of the interconnection of two ports of the same MSTP
device, the device blocks either of the two ports, and the blocked port is the backup port.
•
Edge port: An edge port does not connect to any network device or network segment, but directly
connects to a user host.
•
Master port: A port on the shortest path from the local MST region to the common root bridge. The
master port is a root port on the IST or CIST and still a master port on the other MSTIs.
•
Boundary port: Connects an MST region to another MST region or to an STP/RSTP-running device.
In MSTP calculation, a boundary port’s role on an MSTI is consistent with its role on the CIST. But
that is not true with master ports. A master port on MSTIs is a root port on the CIST.
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