UE sending Registration Request again even after getting rejected with N1 mode not allowed

5G NR
1

Hi Guys,

I am facing one issue in Network. When UE receives N1 mode not allowed, it is sending Registration Request again. I have observed some UEs are doing so even from AMF timer is set for 12 hours until then UE cannot send Registration Request.

2

Following 3GPP 24.501,
#27 (N1 mode not allowed).
The UE capable of S1 mode shall disable the N1 mode capability for both 3GPP access and non-3GPP access
(see subclause 4.9).
4.9.2 Disabling and re-enabling of UE’s N1 mode capability for 3GPP access
When the UE supporting both N1 mode and S1 mode is disabling the N1 mode capability for 3GPP access, it should
proceed as follows:
a) select an E-UTRA cell connected to EPC of the registered PLMN or a PLMN from the list of equivalent
PLMNs;
b) if an E-UTRA cell connected to EPC of the registered PLMN or a PLMN from the list of equivalent PLMNs
cannot be found, the UE may select another RAT of the registered PLMN or a PLMN from the list of equivalent
PLMNs that the UE supports;
c) if another RAT of the registered PLMN or a PLMN from the list of equivalent PLMNs cannot be found, or the UE does not have a registered PLMN, then perform PLMN selection as specified in 3GPP TS 23.122 [5]; or
d) if no other allowed PLMN and RAT combinations are available, then the UE may re-enable the N1 mode capability for 3GPP access and remain camped in NG-RAN of the registered PLMN, and may periodically scan for another PLMN and RAT combination which can provide EPS services or non-EPS services (if the UE supports non-EPS services).

So i think you should check whether other RATs is available on this testing condition. as spec, if no other allowed PLMN and RAT combinations are available, then the UE may re-enable the N1 mode
capability for 3GPP access.

3

The thing is other RAT are available. But UE keep trying to send Registration Request. From MME trace, I can see UE is trying to perform continuous Reselection/Handover after successfully attached to 4G. My understanding is once UE receive CC27, it will not perform registration request again.

Is there any scenario or some config that needs to be checked on NR/ENB side for this?