Method, device, and system for determining route leak

This disclosure discloses a method, device, and system for determining a route leak. The method includes: receiving, by a network monitoring device managing a first autonomous system, an extended BMP packet sent by a first border node of the first autonomous system and an extended BMP packet sent by a second border node of the first autonomous system. The method includes determining, based on the extended BMP packet sent by the first border node and the extended BMP packet sent by the second border node of the first autonomous system, a receiving relationship of a route prefix received by the first border node from the second autonomous system, and a sending relationship of the route prefix sent by the second border node to a third autonomous system. The method further includes determining, based on the receiving relationship and the sending relationship, whether the route prefix is leaked.