Question: In E-UTRAN, when an eNB knows the en-gNB ID but not the TNL address for SCTP connectivity with a candidate en-gNB, what message does the eNB initially send to the MME to request the TNL address?
Option 1: eNB CONFIGURATION UPDATE message
Option 2: eNB ID REQUEST message
Option 3: eNB CONFIGURATION TRANSFER message
Option 4: X2AP EN-DC Configuration Request message
Explanation: When an eNB is aware of the en-gNB ID but lacks the Transport Network Layer (TNL) address necessary for Stream Control Transmission Protocol (SCTP) connectivity with a candidate en-gNB, it utilizes the Configuration Transfer Function. Specifically, the eNB initiates this process by sending an eNB CONFIGURATION TRANSFER message to the Mobility Management Entity (MME), including its own eNB ID and the target en-gNB ID, thereby requesting the TNL address of the candidate en-gNB. This procedure is crucial for establishing connectivity in E-UTRAN, especially in scenarios involving dual connectivity (EN-DC) where seamless communication between different base stations (e.g., eNB and en-gNB) is vital.