HP 1920 Gigabit Ethernet Switch Series User Manual page 201

Figure 177 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
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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
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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,
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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.
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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.
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• Learning—The port learns MAC addresses but does not forward user traffic.
Discarding—The port does not learn MAC addresses or forwards user traffic.
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A port can have different port states in different MSTIs. A port state is not exclusively associated with a
port role. Table 57 lists the port states supported by each port role. (A check mark [√] indicates that the
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