During handover procedure over X2 interface, an identifier is used to exchange control messages between source and target eNodeB. This identifier is known as “UE X2AP ID” and used to distinguish control messages for the UE which is handing over from serving eNodeB to the neighboring eNodeB.
When the serving eNodeB sends Handover Request to the neighboring eNodeB because of poor radio condition or other reason, as well as it adds an ID to the Handover Request message. This ID is called “Old eNodeB UE X2AP ID”. If the feedback from the target eNodeB is positive then the feedback will be Handover Request Acknowledge, and also the target eNodeB will add a new ID to this message which is “New eNodeB UE X2AP ID”.
If first round trip is succeeded, then thereafter control messages will be exchanged between two eNodeB with using these IDs.