HP FlexNetwork NJ5000, User Manual

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Figure 161 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.
•
Master port
—Serves as a port on the shortest path from the local MST region to the common
root bridge. The master port is not always located on the regional root. It is a root port on the IST
or CIST and still a master port on the other MSTIs.
•
Alternate port
—Serves as 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
—Serves as the backup port of a designated port. When the designated port is
invalid, the backup port becomes the new designated port. A loop occurs when two ports of the
same spanning tree device are connected, so the device blocks one of the ports. The blocked
port acts as the backup.
•
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.
Port states
In MSTP, a port can be in one of the following states:
•
Forwarding
—The port learns MAC addresses and forwards user traffic.
•
Learning
—The port learns MAC addresses but does not forward user traffic.
•
Discarding
—The port does not learn MAC addresses or forwards user traffic.
A port can have different port states in different MSTIs. A port state is not exclusively associated with
a port role.
Table 63 lists the port states supported by each port role. (A check mark [ √ ] indicates that
the port state is available for the corresponding port role, and a dash [—] indicates that the port state
is not available for the corresponding port role.)