ISR is nothing but Idle Mode Signalling Reduction Feature which is used to reduce the Re-selection from LTE to UTRAN.
UE can come to a region where LTE is on one side and UTRAN is on another side, this may result UE to select UTRAN and then again LTE by sending RAU and TAU, this may result in a ping pong effect. Thus to reduce this kind of signalling ISR will come into picture. When UE sends RAU to SGSN , SGSN will ask the UE context by sending Context Request to MME , MME will respond back with Context Response with ISR activation flag. Once SGSN receives the response it will include the ISR activation flag to RAU accept and send it to UE. Once UE receives the RAU accpt with ISR flag true, It will store the GUIT from LTE and P-TMSI from UTRAN, Hence UE is now registered to both the Networks.
