TS33.401
7 Security procedures between UE and EPS access network elements
7.2 Handling of user-related keys in E-UTRAN
7.2.7 Key handling for the TAU procedure when registered in E-UTRAN
If the "active flag" is set in the TAU request message or
the MME chooses to establish radio bearers when there is pending downlink
UP data or pending downlink signalling, radio bearers will be established
as part of the TAU procedure and a KeNB derivation is necessary.