Note: This procedure is extra long, I have mentioned important points. It is better that you read it from specification.
The purpose of this procedure is:
-> to modify RRC Connection to establish/modify.release RB.
-> to perform reconfiguration with sync
-> to setup/modify/release measurements
-> to add/modify/release SCells and cell groups.
NAS dedicated information may be transferred from the Network to the UE.
Initiation
When a UE in RRC_CONNECTED mode, network may initiate RRC reconfiguration procedure.
The network applies the procedure as follows:
-> the establishment of RBs is performed only when AS security has been activated
-> the addition of Secondary Cell Group and SCells
Reception of an RRCReconfiguration by the UE
-> if the RRCReconfiguration includes the fullConfig, then perform the full configuration procedure.
-> if the RRCReconfiguration includes the masterCellGroup, then perform the cell group configuration for the received masterCellGroup
-> if the RRCReconfiguration includes the masterKeyUpdate, then perform AS security key update procedure
-> if the RRCReconfiguration includes the secondaryCellGroup, then perform the cell group configuration for the SCG
-> if the RRCReconfiguration message contains the radioBearerConfig, then perform the radio bearer configuration
-> if the RRCReconfiguration message includes the measConfig, then perform the measurement configuration procedure
-> if the RRCReconfiguration message includes the dedicatedNAS-MessageList, then forward each element of the dedicatedNAS-MessageList to upper layers in the same order as listed;
Source TS 38.331