The present disclosure provides a message forwarding method and system, and a relay agent device. The forwarding method includes receiving a first dynamic host configuration protocol (DHCP) for Internet Protocol (IP) version 6 (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 (ID) of the layer-3 interface is added therein, and sending the relay-forward message to a DHCPv6 server.