A method performed, by a user equipment, UE, in a wireless communication system, wherein the UE is a multi radio access technology, RAT, device, the method comprising: transmitting a first user plane, U-plane, data to a secondary radio access technology, RAT, system; transmitting measurement results of the secondary RAT system with identification information of the secondary RAT system to a primary RAT system; receiving a routing configuration modify request from an entity of the primary RAT system in order to modify a routing configuration, wherein the routing configuration modify request includes identity, ID, of a bearer whose routing information is to be modified, a routing type indicating a U-plane aggregation, and a routing rule including a transmission ratio of the primary RAT system to the secondary RAT system; transmitting a response message of the routing configuration modify request to the entity of the primary RAT system in order to indicate either an acceptance or a rejection of the request, wherein the response message further includes a recommended information including a bearer ID, a data flow ID, a routing type, and a routing rule; receiving in response to the response message, from the entity of the first RAT system, a routing configuration modify command in order to modify a routing information of the bearer, modifying the routing information of the bearer based on the received routing configuration modify command and transmitting a second U-plane data to the primary RAT system or to the second RAT system based on applying the transmission ratio according to the modified routing information of the bearer, wherein a handover to the secondary RAT system is performed according to the modified routing information of the bearer, wherein the primary RAT system is a 3rd generation partnership project, 3GPP, long-term evolution, LTE, system, wherein the secondary RAT system is an institute of electrical and electronics engineers, IEEE, 802.11 system, and wherein the ID of the bearer includes an evolved packet system, EPS, bearer ID and a data radio bearer, DRB, ID, and a logical channel ID, LCID. transmitting a first user plane, U-plane, data to a secondary radio access technology, RAT, system; transmitting measurement results of the secondary RAT system with identification information of the secondary RAT system to a primary RAT system; receiving a routing configuration modify request from an entity of the primary RAT system in order to modify a routing configuration, wherein the routing configuration modify request includes identity, ID, of a bearer whose routing information is to be modified, a routing type indicating a U-plane aggregation, and a routing rule including a transmission ratio of the primary RAT system to the secondary RAT system; transmitting a response message of the routing configuration modify request to the entity of the primary RAT system in order to indicate either an acceptance or a rejection of the request, wherein the response message further includes a recommended information including a bearer ID, a data flow ID, a routing type, and a routing rule; receiving in response to the response message, from the entity of the first RAT system, a routing configuration modify command in order to modify a routing information of the bearer, modifying the routing information of the bearer based on the received routing configuration modify command and transmitting a second U-plane data to the primary RAT system or to the second RAT system based on applying the transmission ratio according to the modified routing information of the bearer, wherein a handover to the secondary RAT system is performed according to the modified routing information of the bearer, wherein the primary RAT system is a 3rd generation partnership project, 3GPP, long-term evolution, LTE, system, wherein the secondary RAT system is an institute of electrical and electronics engineers, IEEE, 802.11 system, and wherein the ID of the bearer includes an evolved packet system, EPS, bearer ID and a data radio bearer, DRB, ID, and a logical channel ID, LCID.