Why X2 handover is an optional thing ?

Question

My friend was talking about ANR feature of LTE. By using, ANR feature neighboring eNodeBs update their NRT table. One of my friend was saying that it is not always X2 handover is allowed in X2 link. But he could not tell about cause/reason. I know when X2 handover can not perform, S1-handover shall be executed.

Answer 1

For S1 - Handover all interface will be anyway ready as Source eNB, Source MME, Target eNB, Target MME already get connected (Default connection is required) but to do X2 handover Source eNB and target eNB should be connected previously (Neighbor Discover using ANR) or need to be done x2 when handover is triggered.

So we may spend some time in x2-set up also so s1 handover is by default present and x2 becomes optional and used when x2 is already there or x2- tunnel nfo of target is present.

Also Alkesh is also right that we have a option No X2 Flag in NRT table as part of ANR feature, where it will be set/reset based on the readiness of X2 interface.

Answer 2

On eNB there is usually a flag which indicates that X2 HO should be allowed or not. If it is not allowed, S1 HO will be triggered.
There is flag for X2 link as well so there may be case where X2 link is setup but handover is prohibited

Keeping it optional is up to the operator and how they plan the network.