Question: What action does a target eNodeB (eNB) take when receiving a HANDOVER CANCEL message referencing a non-existent context or containing a Candidate Cells To Be Cancelled List IE under specific conditions?
Option 1: Send an error message back to the source eNB indicating the non-existent context or invalid list.
Option 2: Ignore the entire HANDOVER CANCEL message if the context does not exist, and ignore only the Candidate Cells To Be Cancelled List IE if it's not associated with a conditional handover or was prepared under a different UE-associated signaling connection.
Option 3: Attempt to retrieve the context information from neighboring eNBs before deciding on the next course of action.
Option 4: Automatically trigger a new handover procedure for the affected UE.
Explanation: This is correct because, according to the specifications, if the HANDOVER CANCEL message refers to a non-existent context, the target eNB should ignore the entire message. Additionally, under specific conditions (e.g., no association with a conditional handover or prepared under a different signaling connection), the target eNB is instructed to ignore only the Candidate Cells To Be Cancelled List IE, not the whole message unless the context itself is non-existent. This nuanced response reflects the detailed handling of abnormal conditions in E-UTRAN's X2 Application Protocol.