5GS MM Cause #72: DNN Not Supported or Not Subscribed in the Slice — Field Guide

5GS MM cause #72 (“DNN not supported or not subscribed in the slice”) is returned in a PDU Session Establishment Reject when the SMF (Session Management Function) determines that the requested DNN (Data Network Name — the 5G equivalent of an APN) is not available for the S-NSSAI (network slice) specified in the PDU Session Establishment Request.

This cause code captures a uniquely 5G-SA failure mode: the combination of DNN and S-NSSAI is invalid. Understanding it requires distinguishing between slice registration (Registration Accept with Allowed NSSAI) and session establishment (PDU Session Establishment Accept with DNN + QoS flows).

Technical Reference

How DNN and S-NSSAI Interact in 5G SA

In 5G SA, a PDU session is defined by three parameters:

1. S-NSSAI — identifies the network slice
2. DNN — identifies the data network within the slice
3. SSC mode (Session and Service Continuity mode) — determines session mobility behavior

The UDM stores the UE’s session management subscription as a set of DNN configurations per S-NSSAI. If the UE requests DNN “enterprise.5g” on S-NSSAI SST=2 (URLLC), but the UDM subscription only maps “enterprise.5g” to S-NSSAI SST=1 (eMBB), the SMF returns cause #72.

This DNN-to-slice binding makes 5G session management more granular than LTE, but also introduces a new class of provisioning error specific to the slice-DNN combination.

PDU Session Establishment Flow Where Cause #72 Appears

The flow preceding a cause #72 rejection:

1. UE sends Registration Request with Requested NSSAI (SST=2, SD=0x000001)
2. AMF grants Registration Accept with Allowed NSSAI including SST=2 — the slice is authorized
3. UE sends PDU Session Establishment Request with DNN=“enterprise.5g”, S-NSSAI=SST=2
4. AMF selects an SMF for SST=2 and forwards the PDU Session Establishment
5. SMF queries UDM — finds “enterprise.5g” is not subscribed for SST=2
6. SMF returns PDU Session Establishment Reject with 5GSM cause #72
7. AMF forwards the PDU Session Establishment Reject to the UE

Note: The Registration Accept in step 2 confirms the slice is granted. Cause #72 is not a slice rejection (that would be cause #62) but a within-slice DNN mismatch.

Field Scenarios

DNN not provisioned for the requested slice in the UDM. The UDM subscription maps “internet” DNN to eMBB slice but the device requests “internet” on the URLLC slice. The SMF for URLLC rejects with cause #72. Common during enterprise deployments where the DNN-to-slice mapping was configured incorrectly in the UDM provisioning system.

Private 5G deployment with enterprise DNN on wrong slice. A private 5G network has two slices: eMBB (SST=1) for general connectivity and URLLC (SST=2) for automation control. The enterprise devices are programmed to request the automation DNN (“factory.local”) on the eMBB slice instead of URLLC. Every PDU session attempt receives cause #72 until the device configuration is corrected.

Roaming with unsupported DNN mapping. In 5G roaming, the home PLMN’s DNN may not be mapped to the equivalent DNN in the visited PLMN for the requested slice. The visited SMF cannot establish the PDU session for the home DNN within the slice, returning cause #72.

DNN typo or case-sensitivity error. DNNs in 5G SA are case-insensitive per 3GPP TS 23.003 §9A.2, but some SMF implementations treat them case-sensitively. A DNN “Internet” configured on the device when the UDM expects “internet” can produce cause #72 in non-standard implementations.

Step-by-Step Field Diagnosis

Step 1 — Confirm cause #72 in the PDU Session Establishment Reject. In the 5GS NAS log, locate the PDU Session Establishment Reject (NAS message type 0xC2) and verify the 5GSM cause IE value is 0x48. This is a session management failure, not a registration failure — the UE remains registered.

Step 2 — Read the DNN and S-NSSAI in the PDU Session Establishment Request. In the preceding PDU Session Establishment Request, extract the DNN IE and the S-NSSAI IE. These two values together define the exact combination that was rejected.

Step 3 — Verify the Allowed NSSAI in the Registration Accept. Confirm the rejected S-NSSAI was included in the Allowed NSSAI IE of the Registration Accept. If yes, the slice was granted but the DNN is invalid for that slice. If the S-NSSAI was not in the Allowed NSSAI, the issue is a Registration-level rejection (cause #62) that preceded this event.

Step 4 — Check the UDM subscription for the DNN-S-NSSAI mapping. On the operator or private 5G core side, verify the UE’s session management subscription in the UDM (TS 29.503 §5.2.2.2): confirm the DNN “X” is listed under the subscription context for S-NSSAI SST=Y. A missing entry produces cause #72.

Step 5 — Test with an alternative DNN or S-NSSAI. If the UE can be configured to request the same DNN on a different S-NSSAI, a successful PDU session on the alternative combination confirms the DNN itself is valid but bound to a different slice in the subscription.

Capturing Cause #72 in the Field

5GS NAS PDU Session Establishment Reject messages are captured via DIAG log code 0xB821 (NR5G NAS OTA). The decoded message shows the 5GSM cause IE value 0x48 alongside the PDU Session ID, DNN, and S-NSSAI from the rejected session request. Correlate with the Registration Accept to confirm the slice-level grant preceded the session-level rejection.

Related Cause Codes

• Cause #62 — No network slices available (slice-level rejection, occurs in Registration Reject)
• Cause #67 — Insufficient resources for specific slice and DNN (resources available but exhausted)
• Cause #73 — Insufficient resources for specific slice (slice-level resource exhaustion, no DNN specificity)
• ESM cause #27 in LTE — Missing or unknown APN (the 4G equivalent, without slice dimension)
• Cause #74 — Not supported SSC mode (SSC mode mismatch, not DNN mismatch)