Provided is a communication system which connects a unified C/U router to a network having C/U separation, wherein the stopping of U-plane communication of the unified C/U router is prevented even when the messages transmitted from a C-plane controller of the network having C/U separation do not arrive because of faults in the network having C/U separation. The routing information of a C-plane message (routing protocol) received by a U-plane controller is sent to an adjacent peer without being reflected in a routing table of the U-plane controller. The routing table of the U-plane controller is generated on the basis of the routing information (FIB information) received directly from the C-plane controller. In addition, a keep-alive packet from the unified C/U router is not sent to the C-plane controller and keeps alive the connection with the user router at the transmission destination.