To prevent the occurrence of an unintended routing changeover operation depending on which member link of an LAG link causes a failure when a routing protocol packet (hereinafter described as a packet) is transferred on the LAG link.

A packet is copied and transferred to all member links of the LAG in LAG processing in a transmission-side device and, in LAG processing in a reception-side device, the arrival situation of the packets from the LAG links is confirmed, and whether one of the packets is transferred to upper-layer processing or all of them are discarded is controlled. On the reception-side device, one of the packets is transferred to the upper-layer processing in the case of number of arrival of packets≥minimum number of links, and all the packets are discarded if not. Alternatively, one of the packets is transferred to the upper-layer processing when none of the packets having arrived are lost, and all the packets are discarded if any of the packets is lost.