Embodiments of this disclosure provide a data transmission method, a device, and a system. The method includes: The first Ethernet interface of the first communications device receives a first packet, and determines whether first indication information in the first packet indicates that the first packet is a multi-fed packet. When the first indication information indicates that the first packet is a multi-fed packet, the first communications device drops the first packet. The first Ethernet interface is an Ethernet interface used by the first communications device for connecting to a second communications device.