During Initial attach procedure before AS security establishment eNodeB will enquire about UE capability by sending UECapabilityEnquiry in respond to that UE will send UECapabilityInformation which will carry Eutra capabilities,Geran capabilities,Layer specific capabilities like whether UE supports DRX/SPS/TPC or how many ROHC profiles it supports or whether it supports CA or not,etc.
So according to me UEcapabilityEnquiry is mandatory message during initial attach procedure,same capability information will be forwarded to other eNodeB during handover procedure(intra rat,inter frequency,intra frequency).