The present disclosure provides a message forwarding method and system, and a relay agent device. The forwarding method includes: receiving a first DHCPv6 message from a DHCPv6 client through a layer-3 interface; generating a Relay-forward message, where the first DHCPv6 message is encapsulated into the Relay-forward message and an identifier of the layer-3 interface is added therein; and sending the Relay-forward message to a DHCPv6 server.