（一）协议重建定义
(1)协议3GPP36.331定义触发重建原因包括如下几类
1>upondetecting radio link failure, 2>uponhandover failure,
3>uponmobility from E-UTRA failure,
4>uponintegrity check failure indication from lower layers; 5>uponan RRC connection reconfiguration failure
(2)reconfiguration failure定义：
if the UE is unable to complywith (part of) the configuration included in the RRCConnectionReconfiguration message:
continueusing the configuration used prior to the reception of RRCConnectionReconfiguration message;
if security has not been activated:
performthe actions upon leaving RRC_CONNECTED as specified in 5.3.12, with releasecause other;
else:
initiate theconnection re-establishment procedure as specified in 5.3.7, upon which theconnection reconfiguration procedure ends;
UE在安全模式激活的状态下，如果收到了重配置消息后对于重配置消息内的信元无法匹配/兼容，则发起原因值为“reconfiguration failure”的重建。
(3)handover failure定义
if T304 expires (handover failure): NOTE:
FollowingT304 expiry any dedicated preamble, if provided within the rach-ConfigDedicated, is not available for use by the UE anymore.
UE在切换流程中，在收到了切换的重配置消息之后，会启动T304，但如果在T304超时之前UE无法完成在目标小区的随机接入，则会发起原因值为“handover failure”的重建。
(4)协议3GPP36.331定义重建失败
Upon receiving the RRCConnectionReestablishmentReject message, the UE shall:
performthe actions upon leaving RRC_CONNECTED as specified in 5.3.12, with releasecause 'RRC connection failure';
（5）重建原因（radio link failure）
重建常见原因为RLF, 如果UE检测到当前检测到“radio link failure”，则会发起原因
值为“other”的重建，通常引起RLF存在如下几种机制：
①上行RLC重传达到最大次数
“indication from RLC that the maximum number of retransmissions hasbeen
reached”，包括SRB和DRB，与eNB侧下行的SRB与DRB机制相同，当UE RLC发送了一个PDU之后，需要等到eNB侧反馈对应的状态PDU才能完成一次RLC的正常调度。对于没有收到eNB状态PDU的原因有两个，一个原因为eNB侧上行根本就没有收到任何RLC PDU，也就不会响应状态PDU，另一个原因为eNB响应的状态PDU，由于下行误码的原因，没有到达UE侧。
②MAC层SRI重传达到最大次数
在切换过程中，切换完成命令丢失，此时如果UE有SR发送，因为下行链路问题，UE无法收到ENB的确认，则发起重建请求。