We hear a version of this question often: someone's about to link two NVIDIA DGX Spark units together, sees four USB-C ports staring back at them, and reaches for the USB-C cable already sitting on their desk. Then they find out the port built for the job is one of the two QSFP connectors on the back — and the USB-C port was never meant for this at all.
The DGX Spark packs more port types into one small chassis than most desktop machines, and NVIDIA doesn't spell out what each one is for in a single place. Here's the rundown, and why the QSFP ports are the only correct answer when you're connecting two Sparks together.
What's Actually on the Back of a DGX Spark
| Port | Count | Speed | What it's for |
|---|---|---|---|
| USB-C | 4 (one handles power delivery) | Up to 20–40 Gb/s, depending on which spec sheet you read | Peripherals, external NVMe enclosures, docks |
| RJ-45 Ethernet | 1 | 10 GbE | Standard network and management traffic |
| QSFP (ConnectX-7) | 2 | Up to 200 Gb/s per port | GPU-to-GPU clustering, RDMA, high-throughput storage |
| HDMI | 1 | HDMI 2.1a | Local display output |
Four ports look like peripheral connectivity, one looks like a normal office network jack, and two look unfamiliar unless you've worked with datacenter switches before. That last pair — the QSFP ports — is the one most people skip past, and it's the one that actually matters for anything involving a second Spark.
What the USB-C Ports Are Actually For
DGX Spark's USB-C ports are general-purpose I/O, not a network fabric. Use them for external SSDs, docks, or anything you'd normally plug into a workstation. NVIDIA's own community and third-party reviewers don't even agree on the exact spec — some list the ports as USB4 at up to 40 Gb/s, others measure USB 3.2 Gen 2x2 at 20 Gb/s in practice. Either way, that disagreement doesn't matter much here, because the real gap isn't bandwidth. It's the protocol.
USB-C has no RDMA support. Every byte that crosses it goes through the host CPU and its networking stack. That's fine for copying a model checkpoint to an external drive. It's not fine for the kind of GPU-to-GPU traffic that distributed training and inference generate, where you want data moving directly between GPU memory on each machine with as little CPU involvement as possible.
What the QSFP Ports Are Actually For
Each of the two QSFP ports on the back of a DGX Spark is wired to the onboard NVIDIA ConnectX-7 SmartNIC and runs at up to 200 Gb/s. ConnectX-7 is the same silicon family used in NVIDIA's datacenter networking gear, and it brings the same capability set along with it — including RDMA over Converged Ethernet (RoCE), which is what lets NCCL move tensors between GPUs on separate Sparks without routing everything back through the CPU first.
This is also the port NVIDIA's own documentation and clustering guides point to. When NVIDIA validated a 2-Spark configuration, they did it over a QSFP DAC cable connected directly port-to-port — no switch, no adapters, no USB anywhere in the loop.
So Which Port Do You Actually Use?
It comes down to what you're moving and where it needs to go:
USB-C — external storage, peripherals, a dock for a second monitor and keyboard.
RJ-45 (10GbE) — your existing office or lab network, SSH access, management traffic.
QSFP (ConnectX-7) — connecting a second DGX Spark for clustering, distributed training, or multi-node inference.
If the traffic in question is GPU-to-GPU, the answer is QSFP. There isn't a USB-C configuration that gets you there — it's not a bandwidth shortfall you can work around, it's a missing feature.
The Cable You Need
Two DGX Spark units connect directly to each other over a single QSFP DAC cable — no switch required. NVIDIA's documentation names two approved parts, and we also stock a lower-cost equivalent that matches the port's real-world ceiling exactly:
Part number: MCP1650-V00AE30 — A 0.5-meter, 200G QSFP56 Passive DAC Twinax cable. This is our best-selling option for Spark-to-Spark links: the ConnectX-7 QSFP port tops out at 200 Gb/s regardless of what cable you plug into it, so a QSFP56 cable rated exactly for 200G gets you full performance without paying for headroom you can't use.
If you need to match NVIDIA's exact validated bill of materials — for procurement sign-off or an audit trail — we also stock the two NVIDIA-named parts:
Part number: NJAAKK-N911 (Amphenol) and Part number: LMTQF022-SD-R (Luxshare), both QSFP112, 400G-rated, 0.5 meters, and both explicitly named in NVIDIA's Spark documentation.
All three ship at 0.5 meters on purpose — the two units sit right next to each other on a desk, and a shorter cable means less signal loss and a cleaner connection than a longer run would give you.
Further reading: DGX Spark interconnect reference — approved part numbers, real-world throughput, and what to buy at two, three and four or more nodes.