A method includes receiving, by a first network device, a first packet and a second packet, where the first packet and the second packet belong to first traffic, and all packets included in the first traffic match a first traffic differentiation rule; based on a mapping relationship between the first traffic and a first encryption policy group, encrypting, by the first network device, the first packet using a first encryption policy to obtain a third packet, and encrypting, by the first network device, the second packet using a second encryption policy to obtain a fourth packet, where the first encryption policy group includes the second encryption policy and the first encryption policy, and the first encryption policy and the second encryption policy are different encryption policies; and sending, by the first network device, the third packet and the fourth packet to a second network device.