LTE: How does an eNodeB construct ECGI and TAI for an UE while sending messages like "Initial UE message " ?

Question

Need clarity on following items:
1. What is the use of "UE selected plmn" information for an eNodeB ?
2. Is the ECGI/TAI constructed based on the primary plmn (PLMN broadcasted in 0th index in SIB1 ) or based on the UE selected PLMN ?

Answer 1

The PLMN identity is the one selected by UE during the RRC procedures from the list broadcasted in SIB1.
This way the core network knows which PLMN the UE uses to apply specific authentication (PLMN ID goes into derivation of Kasme in AuC and UE), service and charging profiles.