If you've been researching how to connect two NVIDIA DGX Spark systems, you've likely come across two different cable references: the Q56-200G-CU0-5 (a 200G QSFP56 DAC) and the NJAAKK-N911 (a QSFP112 stacking cable). They are not interchangeable. They use different ports, carry different speeds, and serve different purposes. This article explains exactly what the NJAAKK-N911 is, when you need it, and how it compares to the DAC we covered in our previous guide on DGX Spark interconnects.
What Is the NJAAKK-N911?
The NJAAKK-N911 is NVIDIA's proprietary 400G QSFP112 passive DAC stacking cable designed for the DGX Spark's dedicated stacking port — a separate, purpose-built port distinct from the ConnectX-7 NIC ports used for standard Ethernet networking. It is a 0.5-meter, passive direct-attach copper cable terminated in QSFP112 connectors on both ends.
Part number: NJAAKK-N911
400G QSFP112 Passive Direct Attach Copper Stacking Cable, 0.5m
This cable is used to connect two DGX Spark units via their dedicated stacking ports — the port NVIDIA specifically engineered for high-bandwidth, low-latency GPU-to-GPU communication between two Sparks in a stack configuration.
The Core Difference: Two Ports, Two Purposes
The DGX Spark has two distinct connectivity options relevant to stacking:
1. The ConnectX-7 QSFP56 port — This is a standard Mellanox NIC port operating at 200GbE in Ethernet mode. It's used for general-purpose networking: connecting to a switch, linking to other servers, or pairing two DGX Sparks as a direct Ethernet link. The cable for this port is the Q56-200G-CU0-5 (200G QSFP56 DAC), which we stock and ship same day.
2. The dedicated stacking port — This is a proprietary NVIDIA port engineered specifically for DGX Spark-to-Spark stacking. It operates at 400G using the QSFP112 interface and enables tighter hardware-level integration between two Spark units. The cable for this port is the NJAAKK-N911.
Side-by-Side Comparison
| Feature | Q56-200G-CU0-5 (DAC) | NJAAKK-N911 (Stacking Cable) |
|---|---|---|
| Port used | ConnectX-7 QSFP56 (NIC port) | Dedicated stacking port |
| Connector type | QSFP56 | QSFP112 |
| Speed | 200G | 400G |
| Cable type | Passive DAC twinax | Passive DAC twinax |
| Length | 0.5m | 0.5m |
| Protocol | Ethernet only (no InfiniBand) | Proprietary NVIDIA stacking |
| Use case | General networking, direct Ethernet link | DGX Spark-to-Spark hardware stacking |
| Requires switch? | No (direct connect) | No (direct connect) |
Which One Do You Need?
This is where most people get confused. The short answer:
- If you want to connect two DGX Sparks for distributed AI inference using standard networking frameworks (vLLM, NIM, MPI, NCCL) — you need the Q56-200G-CU0-5 DAC. It uses the ConnectX-7 ports and is compatible with NVIDIA's published DGX Spark Stacking guide.
- If you are using NVIDIA's dedicated hardware stacking feature via the proprietary stacking port — you need the NJAAKK-N911. This is the higher-bandwidth path at 400G, intended for tighter hardware-level coupling between two Spark units.
Many users building a two-Spark inference cluster will start with the Q56-200G-CU0-5 path, since it uses the well-documented ConnectX-7 Ethernet method. The NJAAKK-N911 stacking port path is a distinct, higher-performance option that takes advantage of the dedicated 400G stacking port NVIDIA built into the Spark specifically for scale-out.
Important Limitations to Know
- The NJAAKK-N911 uses a QSFP112 connector — it will not fit the ConnectX-7 QSFP56 port. These are physically distinct connectors; you cannot use one cable in the wrong port.
- NVIDIA currently supports stacking of up to 2 DGX Sparks regardless of which method you use.
- The dedicated stacking port is proprietary to the DGX Spark — the NJAAKK-N911 is not a general-purpose QSFP112 cable and should not be used for switch uplinks or other networking applications.
- Software configuration differs between the two paths — always follow NVIDIA's documentation for the specific port and cable you are using.
Quick Checklist: NJAAKK-N911 Path
- ✅ Two DGX Spark units (Founders Edition or standard)
- ✅ One NJAAKK-N911 cable (0.5m, 400G QSFP112)
- ✅ Both units running DGX OS / Ubuntu 24.04 or later
- ✅ NVIDIA stacking port drivers and firmware current
- ✅ Follow NVIDIA's dedicated stacking port configuration guide
Related Guide
If you're using the ConnectX-7 Ethernet method instead, see our step-by-step guide: How to Connect Two NVIDIA DGX Spark Systems: The Cable You Need and Why — which covers the Q56-200G-CU0-5 DAC, software setup, and configuration checklist.
Shop This Product
NJAAKK-N911 400G QSFP112 Passive DAC Stacking Cable, 0.5m is available from Resilient Tec. View product listing →
For the 200G ConnectX-7 NIC ports on your DGX Spark, see our Q56-200G-CU0-5 200G QSFP56 DAC.