Embodiments provide a notification method, an apparatus, and a communications system, and relate to the communications field, to ensure that a DU performs accurate L2 handling in a CU-DU architecture. The notification method includes: a CU sends a first message to the DU, where the first message includes an ID of a first DRB on which bearer type change occurs and first indication information indicating the DU to perform L2 handling; or the first message includes an ID of a first DRB and second indication information indicating that bearer type change occurs. The CU receives a second message sent by the DU, where the second message includes a cell group configuration of the L2 handling.