Lets first understand why RRCConnectionReestablishment is needed?
This procedure is needed in order to re-establish RRC Connection. Also, it helps to re-establish the SRB1 operation and re-activate the Security algorithms (the security algorithms doesn't change)
Now lets see when it is triggered
1. upon detecting radio link failure
2. upon handover failure
3. upon mobility from E-UTRA failure
4. upon integrity check failure indication from lower layers
5. upon an RRC connection reconfiguration failure
I think name itself tell about the handover failure, lets see what is reconfig failure or what are the cases when reconfig failure is triggered -
The UE receiving RRC Connection Reconfiguration message with any related action i.e. mobility, measurements, physical config, radio config etc will validate a set of necessary conditions. If these conditions are not met then UE will trigger Re-establishment Procedure. There are couple of scenarios where the UE can behave in a different way which are following
1. if the UE is unable to comply with (part of) the configuration included in the RRCConnectionReconfiguration message then roll back to the previous configurations
2. if security has not been activated and UE receives RRCConnectionReconfiguration message then proceed to release the RRC Connection with release cause 'other'
