A troubleshooting method performed by a controller, the method comprising the steps of: a.) determining (steps 205, 410) that a fault occurs on a first multi-layer link passing through a first port on a first network device, wherein the first multi-layer link is a link in a link aggregation group between the first network device and a second network device, ∘ wherein a multi-layer link is a link spanning an Internet Protocol, IP, network and an optical network, and the multi-layer link comprises a cross link, which connects the IP network and the optical network; b.) releasing (steps 210, 415) an optical layer resource of the first multi-layer link, and deleting (steps 215, 420) the first multi-layer link from the link aggregation group; c.) establishing (steps 220, 425) a second multi-layer link used for restoration of the first multi-layer link, based on a first idle port on the first network device and a second idle port on a target network device, wherein: ∘ the first idle port is not the first port, and ∘ the target network device is the second network device when no fault occurs on the second network device or a third network device when the fault occurs on the second network device, wherein the third network device is a backup network device of the second network device; and d.) adding (steps 225, 430) the second multi-layer link to a target link aggregation group between the first network device and the target network device. a.) determining (steps 205, 410) that a fault occurs on a first multi-layer link passing through a first port on a first network device, wherein the first multi-layer link is a link in a link aggregation group between the first network device and a second network device, ∘ wherein a multi-layer link is a link spanning an Internet Protocol, IP, network and an optical network, and the multi-layer link comprises a cross link, which connects the IP network and the optical network; ∘ wherein a multi-layer link is a link spanning an Internet Protocol, IP, network and an optical network, and the multi-layer link comprises a cross link, which connects the IP network and the optical network; b.) releasing (steps 210, 415) an optical layer resource of the first multi-layer link, and deleting (steps 215, 420) the first multi-layer link from the link aggregation group; c.) establishing (steps 220, 425) a second multi-layer link used for restoration of the first multi-layer link, based on a first idle port on the first network device and a second idle port on a target network device, wherein: ∘ the first idle port is not the first port, and ∘ the target network device is the second network device when no fault occurs on the second network device or a third network device when the fault occurs on the second network device, wherein the third network device is a backup network device of the second network device; and ∘ the first idle port is not the first port, and ∘ the target network device is the second network device when no fault occurs on the second network device or a third network device when the fault occurs on the second network device, wherein the third network device is a backup network device of the second network device; and d.) adding (steps 225, 430) the second multi-layer link to a target link aggregation group between the first network device and the target network device. <EOS>