Huawei’s troubleshooting guidance for iSCSI Link Failure states that when the running status of an iSCSI host/front-end port is “Link down” and the link indicator is steady red or off , the fault is investigated as a physical/link-layer problem . Huawei’s documented possible causes include items such as cables between the host and storage being removed or damaged , optical module faults/incompatibility , or front-end port faults .

Therefore, A can be a possible cause because Huawei has a specific fault case for an incompatible optical module type causing the port to be unavailable. C can also be a possible cause because Huawei explicitly lists cables removed or damaged/loose as a cause of the link-down condition.

B and D are not possible causes of this specific exception. Incorrect IP settings on either the storage iSCSI front-end port or the application server service network port are network-layer configuration issues , but they do not cause the physical link indicator to turn steady red or the port running status to become Link down . A physical link-down alarm points to hardware, cabling, or optical-module problems instead.