Short answer: passive DAC copper cables are reliable up to about 3 meters at 25G and 100G, and up to roughly 1-2 meters at 400G. Beyond that, signal loss increases enough that most switch and NIC vendors won't guarantee a stable link, and you should move to an active optical cable (AOC) or a fiber transceiver pair instead.
This distance ceiling catches a lot of homelab and lab builders off guard, especially when scaling from a simple 2-node setup to a rack that spans more than a couple of feet. Forum threads asking why a passive DAC won't hold a link over 10 meters are common, and the answer is almost always that the cable was pushed well past its supported range.
Why Passive Copper Has a Hard Distance Ceiling
A passive DAC cable is just twinax copper with a transceiver housing molded onto each end, so there is no signal regeneration anywhere along the link. As the electrical signal travels down the copper, it loses energy and picks up noise, an effect called insertion loss. The faster the data rate, the less margin there is to absorb that loss before the receiver can no longer reliably tell a 1 from a 0. That is why the maximum usable length keeps shrinking as speeds climb from 10G to 100G to 400G.
Typical maximum passive DAC lengths by speed:
- 10GbE (SFP+): typically reliable up to 7-10 meters passive
- 25GbE/28G (SFP28): typically reliable up to 3-5 meters passive
- 100GbE (QSFP28): typically reliable up to 2-3 meters passive
- 400GbE (QSFP-DD/OSFP): typically reliable up to 1-2 meters passive
These are general guidelines, not hard specs. Exact supported lengths vary by vendor and by the quality of the cable's copper and shielding, so always check the switch or NIC vendor's own compatibility list for the exact part number.
What Happens When You Push a Passive DAC Past Its Limit
When a passive DAC is run beyond its supported distance, the symptoms usually look like an intermittent or partial fault rather than a clean failure, which is what makes it confusing to diagnose:
- Link never comes up at all, even though the module is detected
- Link comes up but flaps intermittently, especially under load or as temperature changes
- Link holds at a reduced negotiated speed instead of the full rated speed
- Elevated CRC errors and packet retransmits even when the link appears stable
DAC vs. AOC: What Actually Changes
An active optical cable looks like a DAC from the outside, fixed ends, no separate optics to source, but it converts the signal to light internally, which removes the copper distance penalty entirely. Here is what actually changes when you move from DAC to AOC:
- Distance: DAC tops out at a few meters depending on speed; AOC reliably runs 10-100 meters since it is not limited by copper insertion loss
- Cost: DAC is cheaper per unit since it has no optical engine; AOC costs more but is still less than separate transceivers plus fiber patch cables
- Power draw: DAC draws little to no additional power; AOC draws more since each end has an active optical engine
- Point of failure: DAC has nothing to fail optically, just copper and connectors; AOC adds an optical engine at each end that can fail independently of the fiber itself
Choosing the Right Cable for Your Build
- Same rack, adjacent nodes: passive DAC is the cheapest and most reliable option
- Adjacent rack or across a row: AOC, or a longer passive DAC only if it falls within the vendor's supported range for that speed
- Across a room, floor, or building: separate fiber optic transceivers and patch cabling, not a fixed AOC, since AOC is still a fixed-length cable rather than a flexible fiber run
Resilient Tec stocks passive DAC and AOC in the exact lengths that match real deployments, so you are not guessing at whether a length is inside or outside the supported range for your speed.
FAQ
Can I use a passive DAC cable for 10 meters at 25G? No. Passive DAC at 25G is only reliable up to about 3-5 meters depending on the vendor; at 10 meters, use an AOC or a fiber transceiver pair instead.
Does a longer passive DAC need more power? No, passive DAC draws negligible power at any supported length; the issue with longer runs is signal integrity, not power.
Is AOC as reliable as DAC? Yes, for its intended distance range, though AOC has an added optical engine at each end that is a second potential point of failure compared to a passive copper link.
Can I mix DAC and AOC in the same rack? Yes. It is common to use passive DAC for short in-rack links and AOC for longer runs to other racks within the same deployment.