A control apparatus sets, in a relay device: first control information that forwards, when a packet for flooding is received by a representative port of a trunk, the packet to a prescribed forwarding destination port and to a representative port of another trunk; second control information that executes, when the packet for flooding is received by a nonrepresentative port of the truck, forwarding of the packet to a stack link port associated with the same trunk group as the trunk of the port that received the packet for flooding, and forwarding to a nonrepresentative port of another trunk in the trunk group; and third control information that forwards, when the packet for flooding is received by the stack link port, the packet to a prescribed forwarding destination port and to a representative port of a trunk not belonging to the same trunk group.