A radio communication system comprising: a first radio station configured to operate a first cell; a second radio station configured to operate a second cell; and a radio terminal capable of aggregating the first cell as a primary cell with the second cell as a secondary cell, wherein the first radio station is configured to send, to the second radio station, a first request for the second radio station, wherein the first request is configured to request the second radio station to prepare radio resources for aggregating the first cell with the second cell by the radio terminal, while an established first radio connection between the first radio station and the radio terminal is maintained on the first cell, and wherein the first request includes at least E-UTRAN Radio Access Bearer Identity, E-RAB ID, which indicates an identity of an E-UTRAN radio access bearer between the radio terminal and a Serving Gateway, S-GW, via the second radio station; the second radio station is configured to send, to the first radio station, radio resource configuration information of the second cell, and the first radio station is configured to transmit, to the radio terminal, a radio connection configuration information message including at least the radio resource configuration information of the second cell while the established first radio connection between the first radio station and the radio terminal is maintained, wherein the radio resource configuration information of the second cell includes a Data Radio Bearer Identity, drb-identity, which indicates an identity of a data radio bearer between the radio terminal and the second radio station, mapped on the E-UTRAN radio access bearer indicated by the E-RAB ID. a first radio station configured to operate a first cell; a second radio station configured to operate a second cell; and a radio terminal capable of aggregating the first cell as a primary cell with the second cell as a secondary cell, wherein the first radio station is configured to send, to the second radio station, a first request for the second radio station, wherein the first request is configured to request the second radio station to prepare radio resources for aggregating the first cell with the second cell by the radio terminal, while an established first radio connection between the first radio station and the radio terminal is maintained on the first cell, and wherein the first request includes at least E-UTRAN Radio Access Bearer Identity, E-RAB ID, which indicates an identity of an E-UTRAN radio access bearer between the radio terminal and a Serving Gateway, S-GW, via the second radio station; the second radio station is configured to send, to the first radio station, radio resource configuration information of the second cell, and the first radio station is configured to transmit, to the radio terminal, a radio connection configuration information message including at least the radio resource configuration information of the second cell while the established first radio connection between the first radio station and the radio terminal is maintained, wherein the radio resource configuration information of the second cell includes a Data Radio Bearer Identity, drb-identity, which indicates an identity of a data radio bearer between the radio terminal and the second radio station, mapped on the E-UTRAN radio access bearer indicated by the E-RAB ID.