Network system, control method thereof, and controller

Provided are a network system, a control method thereof, and a controller where at least one relay processing device is allocated to each server. Further, a plurality of switches are arranged in the network, and a controller controls the switches. Specifically, in response to a request from a request source switch, the controller designs a flow passage from the request source switch to a destination address. At this time, the controller refers to allocation information indicating the allocation relationship between the server and the relay processing device, selects one relay processing device as a target among the relay processing devices allocated to the server designated by the destination address, and designs a passage including the target. Then, the controller instructs each switch on the designed passage to establish a flow table so that a packet is transferred along the designed passage.