raffic load considerations. For instance, a UE config- ured to perform NR measurements may eventually report a B1 event (triggered when a neighboring inter-system cell becomes better than a threshold) to the serving eNB (MN). 2. If the MN decides to initiate the Secondary Node Addition procedure, it sends the X2-AP SgNB Addition Request message requesting the SN to allocate the needed resources, for the DRBs and possibly also for the split SRB operation, also providing information about the usable UE capabilities. The MN also provides the latest measurement results for the SN to choose and configure the SCG cell(s). For a specific E-RAB, the MN may request the direct establishment of an SCG or a split bearer, without first having to establish an MCG bearer. It is also possible that all E-RABs can be configured as SN terminated bearers. The SN performs admission control and allocates the necessary radio resources and, depending on the bearer option, the respective transport network resources. For bearers requiring SCG radio resources, the SN decides the PSCell and other SCG SCells and triggers Random Access SO that synchronization of the SN radio resource configuration can be performed. 3. If the SN decides to admit the request, it responds with the X2-AP SgNB Addition Request Acknowledge message providing the resource configuration to the MN in an NR RRC configuration container. 4. The MN sends the RRCConnectionReconfiguration message to the UE with the NR RRC configuration received from the SN. 1.SgNB Addition Request 2. SgNB Addition Request Acknowledge 3. RRCConnectionReconfiguration 4. RRCConnectionReconfiguration Complete 5. SgNB Reconfiguration Complete 6. Random Access Procedure 7. SN Status Transfer 8. Data Forwarding 9. Path Update procedure Figure 4.3.12 Secondary Node Addition procedure. (Source: Reproduced by permission of © 3GPP). 4.3 Multi-Radio Dual Connectivity 5. The UE replies with an aRRCConnectionReconfigurationComplete message, which may also contain a NR RRC response message. 6. The MN sends