A method performed by a network device that taps to a network having a routing device, includes: receiving a first packet tapped from the network; determining a first information regarding an input interface of the routing device based on a destination address of the first packet; receiving a second packet tapped from the network; determining a second information regarding an output interface of the routing device based on a source address of the second packet; determining a first CRC for the first packet; determining a second CRC for the second packet; and comparing the first CRC with the second CRC at the network device to determine whether the first packet and the second packet are the same.