H3C Technologies H3C S12500-X Series Switches - Manual (Page 82)

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Port roles
A port can play different roles in different MSTIs. As shown in Figure 20 , 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 20 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—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.
• Edge port—Does not connect to any network device or network segment, but directly connects to a
user host.
• 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.
• 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: