Various embodiments provide a fault diagnosis method and an apparatus. In those embodiments, a centralized management apparatus receives a first packet from a first network device by using a first IGP monitoring protocol session, where the first packet includes a first message sent by the first network device to a second network device. The centralized management apparatus receives a second packet from the second network device through a second IGP monitoring protocol session, where the second packet includes a second message sent by the second network device to the first network device. The centralized management apparatus performs fault diagnosis based on the first packet and the second packet.