How can the eNB fill the GUMMEI of X2 handover signaling of Handover Request

4G LTE
1

As we know, when the X2 handover happens, the source eNB will send Handover Request to the target eNB. from 3GPP 36.423 document we can see, the GUMMEI item is mandatory, my question is: how can eNB get this GUMMEI?

  1. S1 setup process, there are two lists: MME GROUP ID list and MMEC list
  2. Initial UE Message, the GUMMEI is optional
  3. Initial Context Setup Request message, the GUMMEI is optional

if both 2 and 3 two messages have no GUMMEI, how does eNB fill the mandatory item GUMMEI of Handover Request once a X2 handover is triggered? Select one from upper 1? how to select?