During initial attach At what stage MME came to know about IMSI of the UE?

Question

After attach procedure shut down enodeb only and again run enodeb then how it will get connect to ue(through which message enb will get ue context).

Answer 1

1)In intialUEmessage(Attach request,PDN Connectivity request) we have 5 main IE's they are
"eNB UES1 AP ID,TAI, ECGI, NAS PDU,RRC -Establishment Cause".

2)in that The NAS PDU contains 4 more parameters like "EPS mobile id,ue network capability,ESM message container and TMSI status"

3)here from "EPS mobile ID" the MME can know the IMSI value(15bits)

Answer 2

Just to add what Siva mentioned, If MME receives "EPS mobile ID" other than IMSI, MME can ask about IMSI from UE using a procedure named "Identification procedure". For more details of Identification procedure, please go through 24.301 spec.