The eNB maintains a local mapping between its eNB UE S1AP ID and the MME’s MME UE S1AP ID . If the MME sends a message (e.g., UE CONTEXT RELEASE COMMAND , DOWNLINK NAS TRANSPORT ) with an ID that the eNB has no record of, the eNB rejects it with this cause.
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. unknown-mme-ue-s1ap-id
| 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. | The eNB maintains a local mapping between its