Abstract: A 5G gNodeB has a Centralized Unit (CU) and one or more Distributed Units (DU). Each of the one or more DUs has a cell topology information that it sends to the CU so that the CU has information for each DU for each of the DU's cells, including which cells are available to each primary cell for carrier aggregation. When a UE connects to the CU, the CU queries its configuration information—including cell topology information for the DU connected to the UE— to determine if and which secondary cells are available for carrier aggregation. If so, the CU may signal the DU to configure and subsequently activate the one or more secondary cells to which the UE can connect for carrier aggregation.