| Data Source | What to Look For | |-------------|------------------| | | Find the MME UE S1AP ID in the failing message. Search backwards for a UE CONTEXT RELEASE COMPLETE or UE CONTEXT RELEASE REQUEST for the same ID. | | eNB Logs | Look for S1AP: Unknown MME UE S1AP ID or No active UE context for MME ID X . | | MME Logs | Check timestamps of the last successful INITIAL CONTEXT SETUP for that UE. See if a RESET message was sent/received. |
In LTE and 5G (NSA) S1AP signaling, the MME UE S1AP ID is a critical identifier used by the MME to uniquely address a specific UE context over the S1 interface. When an eNB receives a message containing an ID it does not recognize, it responds with a cause = unknown-mme-ue-s1ap-id . This post breaks down why this happens and how to fix it. unknown-mme-ue-s1ap-id
MME -> eNB: UE CONTEXT RELEASE COMMAND (MME UE S1AP ID = 12345) eNB -> MME: ERROR INDICATION (Cause: unknown-mme-ue-s1ap-id) Root cause: eNB log shows [ERROR] UE context 12345 already freed due to RRC timeout 10ms earlier . | Data Source | What to Look For
unknown-mme-ue-s1ap-id is almost always a transient state mismatch between core and RAN. It does not indicate a security issue or permanent fault, but frequent occurrences point to poor S1 link reliability or mismatched timeout values. Always correlate eNB and MME timestamps to the millisecond level. | | MME Logs | Check timestamps of
Troubleshooting unknown-mme-ue-s1ap-id : Causes, Impacts, and Resolution