After doing attach, when an UE moves into the IDLE mode , it may perform cell re-selection . UE changes its current serving cell when it finds the current serving cell is not good enough. Intra frequency cell re-selection parameters are broadcasted in SIB3 and SIB4. SIB3 mainly contains the serving cell specific parameters that used for cell re-selection while SIB4 contains neighbor cell selection related parameters.
Before selecting a cell, UE detects all the cells and ranked them including serving cell. It selects the top ranked cell.
If the new cell selected by an UE belongs to a different tracking area . I meant to say tracking are that is being broadcasted in the new cell is not the one of those present in the TAI list , UE perform Tracking area update procedure. So that MME can track UE in the idle mode.
