To rectify the unidirectional communication fault on an Eth-Trunk, the following features are required:
- 802.3ah: The link connectivity can be detected through 802.3ah. When a unidirectional communication fault occurs, the status of the two ends of the trunk can be the same in time.

- LACP: Through the exchange of LACPDUs, LACP can ensure that the status of the two ends of the trunk is the same. When a unidirectional communication fault occurs, LACP can detect the fault in time and transfer the selected status to the other side, thus solving the packet loss problem caused by the unidirectional communication fault.

ps. In V100R005 and later versions, DLDP can monitor the link status of optical fibers or copper twisted-pair cables. If a unidirectional link exists, DLDP automatically disables the interface or prompts the user to manually disable the interface. This prevents network faults.

Some Questions and answers )

What is DLDP?

The Device Link Detection Protocol (DLDP) monitors the link status of optical fibers or copper twisted-pair cables such as super Category 5 twisted pairs. If a unidirectional link is found on an interface, DLDP automatically shuts down or requests users to manually shut down the interface. This prevents network faults.

Working Process of DLDP

If a link of the DLDP-enabled interface is in Up state, the DLDP-enabled interface sends DLDP
packets to the peer device and processes the DLDP packets received from the peer device. DLDP
packets sent vary with DLDP states.

DLDP State	Types of DLDP Packets Sent
Active	Advertisement packets with RSY tags.
Advertisement	Common Advertisement packets.
Probe	Probe packets.
Disable	Disable packets. After one Disable packet is sent, a RecoverProbe packet is sent.

 
A received DLDP packet is processed as follows:
- In authentication modes, the DLDP packet is authenticated. The DLDP packet is discarded if it fails to pass the authentication.
- The DLDP packet is discarded if the interval for sending Advertisement packets in the DLDP packet is different from that on the local device.
- The DLDP packet is processed.

If no echo packet is received from the neighbor, DLDP performs the following operations

In normal mode, no echo packet is received when the Echo timer expires.	The DLDP-enabled interface transits to the Disable state, and displays logs and information about traced packets and sends Flush packets. In addition, the local interface is manually or automatically shut down according to the DLDP Down mode and the neighbor entry is deleted.
In enhanced mode, no echo packet is received when the enhanced timer expires.	The DLDP-enabled interface transits to the Disable state, and displays logs and information about traced packets and sends Flush packets. In addition, the local interface is manually or automatically shut down according to the DLDP Down mode and the neighbor entry is deleted.

Link Auto-recovery Mechanism

If the interface shutdown mode is set to auto, DLDP sets the interface where a unidirectional link is detected to DLDP Down automatically. A DLDP Down interface cannot forward service traffic or send/receive any protocol packets except DLDPDUs. A DLDP Down interface can be recovered before link recovery. The DLDP Down interface sends RecoverProbe packets periodically. If correct RecoverEcho packets are received, it indicates that the unidirectional link changes to the bidirectional link and the DLDP Down interface becomes Up. The detailed process is described as follows: The DLDP Down interface sends a RecoverProbe packet, which carries information about the local interface, every two seconds. When receiving the RecoverProbe packet, the peer end returns a RecoverEcho packet. When receiving the RecoverEcho packet, the local interface checks whether the neighbor information in the RecoverEcho packet is the same as that on the local interface. If they are the same, the link between the local interface and the neighbor is considered to have been restored. The local interface transits from Disable state to Active state and the neighbor relationship is set up. Only DLDP Down interfaces can send and process Recover packets, including RecoverProbe packets and RecoverEcho packets. The auto-recovery mechanism does not take effect on interfaces that are manually shut down.