LTE: When does an eNodeB send Error Indication message to MME for an UE ?

Question

From MME to eNodeB, UE context modification procedure is triggered I know but what makes trigger of UE Context modification procedure from eNodeB to MME ?

Answer 1

In simple words:

eNodeB should send Error Indication to MME for either of the following cases:

1. When MME sends a message to eNB for a UE with wrong or missing or invalid enb-ue-s1ap-id/mme-ue-s1ap-id
2. When MME sends a message to eNB for a UE with wrong values for any mandatory IE

Answer 2

As per definition presents in the specification, when a node (either eNodeB or MME) receives an incoming message, and finds an error then it responds back to the sender with the Error Indication message.
In an online reference , I saw a call flow in which error indication message was being sent from eNodeB to MME when eNodeB has encountered an issue related to radio interface for an UE.