Message forwarding method, system, and relay agent device

A message forwarding method, comprising: receiving (41), by means of a layer 3 relay agent device of the Dynamic Server Configuration Protocol for Internet Protocol version 6, DHCPv6, a first DHCPv6 message from a DHCPv6 client via a layer 3 interface of the DHCPv6 layer 3 relay agent device; generate (41), by means of the DHCPv6 layer 3 relay agent device, a relay forwarding message, where the relay forwarding message comprises two interface ID identifier option fields, where one of the two fields of identifier option interface ID and the first DHCPv6 message are encapsulated in an inner layer of the relay forwarding message, and another of the two second interface ID identifier option fields is encapsulated in an outer layer of the relay forwarding message, where one of the two interface ID identifier option fields indicates an identifier of a port on the interface device. DHCPv6 Layer 3 Relay Agent where the first DHCPv6 message is received, and another of the two interface ID identifier option fields indicates an identifier of the layer 3 interface of the DHCPv6 layer 3 relay agent device through which the first DHCPv6 message is received; and sending (41), by the DHCPv6 layer 3 relay agent device, the relay forwarding message to a DHCPv6 server.