Provided are method for extending a bridge domain of a first overlay SDN network to a second overlay SDN network using a repeater, the method comprising registering a relay switch of repeater as a switch included in first overlay SDN network and second overlay SDN network; generating a tunnel which connects relay switch with switches included in first overlay SDN network; generating a tunnel which connects the relay switch with switches included in second overlay SDN network; receiving route information for being connected to a virtual machine (VM) belonging to first overlay SDN network, from a first SDN control unit of first overlay SDN network; converting route information; and transmitting converted route information to second SDN control unit.