An error code information transfer method, a network device and a communication system, which are used for solving the problems that the existing BFD mechanism cannot be applied to a layer 2 network and the range of application is not wide enough. In some feasible embodiments of the present invention, the method comprises: a second network device detecting whether an error code is generated in a first unidirectional link from a first network device to the second network device; when it is detected that the error code is generated in the first unidirectional link, generating an ISIS message comprising error code information, wherein the error code information is used for indicating that the error code is generated in the first unidirectional link; and sending the ISIS message to the first network device.