Port states, How mstp works – H3C Technologies H3C WX5500E Series Access Controllers User Manual
Page 76
65
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 is invalid, the
backup port becomes the new designated port. A loop occurs when two ports of the same
spanning tree device are interconnected, so the device blocks one of the ports. The blocked port
acts as the backup.
•
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 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:
•
Forwarding—The port receives and sends BPDUs, learns MAC addresses, and forwards user
traffic.
•
Learning—The port receives and sends BPDUs, learns MAC addresses, but does not forward user
traffic. Learning is an intermediate port state.
•
Discarding—The port receives and sends BPDUs, but does not learn MAC addresses or forward
user traffic.
When in different MSTIs, a port can be in different states.
A port state is not exclusively associated with a port role.
lists port states that each port role
supports. A check mark [√] indicates that the port supports this state, while a dash [—] indicates that the
port does not support this state.
Table 12 Port states that different port roles support
Port role (right)
Port state (below)
Root port/master
port
Designated port Alternate port
Backup port
Forwarding
√
√ — —
Learning
√
√ — —
Discarding
√
√
√
√
How MSTP works
MSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculated
CST. Inside an MST region, multiple spanning trees are calculated. Each spanning tree is called an
"MSTI." Among these MSTIs, MSTI 0 is the IST.