H3C S7500E-X Configuration Manual page 82

Port roles
A port can play different roles in different MSTIs. As shown in
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 23 Port roles
MST region
Device B
Port B2
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—Acts 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—Acts 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—Acts 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. However, 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 receives and sends BPDUs, learns MAC addresses, and forwards user
traffic.
To the common root
Port A1 Port A2
Port A3 Port A4
Device A
(Root bridge)
Port B1
Port B3
Port C1
Port C2
Device C
Port C3 Port C4
To other MST regions
Device D
Port D1
Port D2
Port D3
69
Figure 23, an MST region contains
Root port
Designated port
Alternate port
Backup port
Edge port
Master port
Boundary port
Normal link
Blocked link