MME -> eNB: UE CONTEXT RELEASE COMMAND (MME UE S1AP ID = 12345) eNB -> MME: ERROR INDICATION (Cause: unknown-mme-ue-s1ap-id)

For 5G networks, while the S1AP protocol is replaced by NGAP (Next Generation Application Protocol), the concept of an AMF UE NGAP ID and a similar error cause ( unknown-amf-ue-ngap-id ) exists. The lessons learned from troubleshooting unknown-mme-ue-s1ap-id in LTE remain directly applicable to 5G Standalone (SA) deployments.

However, network engineers and protocol analysts occasionally encounter a frustrating error: . This message, often found in MME logs, pcap traces, or eNodeB alarm logs, indicates a critical state mismatch between the radio access network (RAN) and the core network. When this error occurs, the MME essentially tells the eNodeB: “You’re referring to a user session that I have no knowledge of or that no longer exists.”

| 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. |

Review the inactivity timers on both the eNodeB and the MME. If the MME's timer is shorter than the eNodeB's, it will drop the connection while the base station still thinks it's active.