If you're building an AI or local-LLM cluster around Mellanox/NVIDIA ConnectX cards and just need to know which cable to buy: it's a 100G QSFP28 passive DAC (Direct Attach Copper) cable, in 1 meter for node-to-node links inside the same rack, or 3 meters if your nodes are spread across racks or need to reach a top-of-rack switch. Below is the exact part number, why 100G ConnectX has become the go-to backbone for homelab AI clusters, and what to check before you order.
Why 100G ConnectX Networking Is Taking Over the AI Homelab
Two things happened at once: ConnectX-4 and ConnectX-5 100GbE network cards flooded the used market as hyperscale data centers upgraded to faster gear, and local LLM inference/training on multi-node, multi-GPU rigs went mainstream. That combination means a homelab builder can pick up a 100GbE-capable NIC for a fraction of its original price, but still needs a reliable, low-latency link to move model weights, KV-cache, and tensor-parallel traffic between nodes. A passive copper DAC cable is the cheapest and most reliable way to make that link over short distances, which is why it shows up constantly in homelab and local-LLM cluster build threads.
What Is a 100G QSFP28 DAC Cable?
A DAC (Direct Attach Copper) cable is a fixed twinax cable with a QSFP28 transceiver molded onto each end, no separate optics, no fiber, and no transceiver to fail or overheat. It's the standard way to connect two 100GbE ports over distances up to a few meters, and it draws less power and costs less than an active optical cable (AOC) at the same data rate.
| Spec | Detail |
|---|---|
| Data rate | 100GbE |
| Connector | QSFP28 to QSFP28 |
| Cable type | Passive copper twinax (DAC) |
| Standards | QSFP28 MSA, IEEE 802.3bj |
| Typical lengths | 0.5m to 3m (passive range) |
| Compatible platforms | Mellanox/NVIDIA ConnectX-4/5/6, Cisco, Arista, Juniper, Dell |
Which Cable and Length Do You Actually Need?
Part number: MCP1600-C001 is a 1-meter, 100G QSFP28 passive DAC cable. This is the right choice for connecting two nodes sitting in the same rack or on the same desk/shelf, which covers most 2-node and small multi-node homelab clusters.
Part number: MCP1600-C003 is the 3-meter version, for nodes in separate racks, or when you need to reach a top-of-rack switch instead of direct-connecting two nodes.
Both are pre-programmed to work with Mellanox/NVIDIA ConnectX cards out of the box, and are backed by a lifetime warranty.
Compatibility: ConnectX, Cisco, Arista, Juniper, Dell
These cables use the same QSFP28 MSA electrical and mechanical spec regardless of vendor, so a single SKU works whether your NICs are Mellanox/NVIDIA ConnectX-4/5/6, or your switch is Cisco, Arista, Juniper, or Dell. That means you can mix a ConnectX NIC on one end with a switch from a different vendor on the other end without buying separate vendor-locked cables.
Already Running a DGX Spark Cluster? Here's How 100G Fits In
If you've already got a DGX Spark or DGX Spark-class cluster (Spark, ASUS Ascent GX10, Dell Pro Max, and similar GB10-based systems all share the same ConnectX-7 networking), a 100G QSFP28 DAC cable is a straightforward way to add a storage server, a GPU rig, or an extra compute node into that same network without needing a QSFP112 stacking cable for every connection.
FAQ
Is a 100G QSFP28 DAC cable compatible with Mellanox ConnectX-4, ConnectX-5, and ConnectX-6?
Yes. ConnectX-4 and ConnectX-5 cards use native QSFP28 100GbE ports, and ConnectX-6 cards are backward compatible with QSFP28 DAC cables at 100G speeds.
What length DAC cable do I need for a 2-node AI cluster?
A 1-meter cable covers most 2-node setups where both machines sit in the same rack or on the same desk. Go with 3 meters if the nodes are in separate racks or you're routing through a switch.
Can I use a 100G DAC cable with a DGX Spark?
DGX Spark's ConnectX-7 ports are validated for a QSFP112 stacking cable when linking two Sparks directly. A 100G QSFP28 DAC cable is the right choice when you're adding a separate 100G-networked machine, such as a NAS, switch, or another server, alongside your Spark cluster rather than linking Sparks to each other.
DAC vs. AOC for 100G homelab networking, which is better?
For runs under 3 meters, passive DAC is cheaper, uses less power, and has one less component that can fail since there's no optical engine. AOC only makes sense once you need cable lengths beyond what passive copper supports.
Do I need a switch, or can I direct-connect two nodes?
For a simple 2-node cluster, you can direct-connect the two NICs with a DAC cable and no switch at all. Once you go beyond two nodes, you'll need a 100G-capable switch, or NIC-based bonding, to fan out the connections.
Bottom Line
If you're networking a ConnectX-based AI or local-LLM homelab cluster, a 100G QSFP28 passive DAC cable is the cheapest, most reliable way to link your nodes. Grab the 1-meter MCP1600-C001 for same-rack builds, or the 3-meter MCP1600-C003 if you need the extra reach.