LTE: When two eNB have different plmn id why handover is only s1 basesd handover?

Question

I found two MME UE S1-AP IDs i.e. MME UE S1AP ID (Mandatory) and MME UE S1AP ID 2 (optional).
I got why mandatory one is present but what's use of optional one.

Can someone please explain it ?

Answer 1

In lab environment it is possible to establish X2 link between two eNodeBs boardcasting the different PLMNs and X2 handover can be executed. But in the real deployment mostly only one PLMN is associated with one MME/Operator that's why both the eNodeB would be connected to different MMEs.
In this scenario only S1-based handover can be executed. In essence, broadcasting of different PLMN does not restrict execution of X2 handover.