S1-AP handover required message carries "Target ID". This IE contains sub-elements like target "global enodeb Id" and "tracking area code". When source MME receives handover required message for an UE, it check whether it serves target eNodeB or not.
If MME serves target eNodeB then it shall prepare and send handover request message to target eNodeB. If it doesn't then based on tracking area code of target cell, source MME prepares and sends Forward Relocation Request to target MME.