A Software Defined Network (SDN) controller receives a detecting packet reported by a first switch and initiated by a first device. The SDN controller sends, for each of switches having an SDN connection with the SDN controller, the detecting packet and information of all downlink ports of the switch to the switch in a way that the switch is capable of sending the detecting packet through the downlink port. The SDN controller receives a response packet which is sent by a second switch and initially generated by a second device in response to receiving the detecting packet from the second switch, wherein an IP address of the second device matches a destination IP address of the detecting packet. The SDN controller sends forwarding configuration information to the second switch in a way that the second switch is capable of configuring a port through which the response packet is received.