EPSFB TAU getting rejected with cause: 9 (UE identity cannot be derived by the network)

Mobility
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1

Dear Experts.

5G-4G EPSFB TAU in LTE is getting rejected with cause: 9 (UE identity cannot be derived by the network).

UE Is going into idle mode post TAU Reject after EPSFB.

What can be the reason for TAU reject in this case and how to rectify the same?

2

Check RRC connection release should be: load balancing TAU.

3

RRC Connection release is with cause: other.

4

Before TAU request, check RRC connection release, what cause it has?

5

Is n26 supported?

Possible reason for reject is that MME does not have context for this UE in absence of n26.