H3c-technologies H3C WX5500E Series Access Controllers Uživatelský manuál stáhnout pdf (Strana 76)

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. Table 12 lists por

t 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.

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