QSFP-100G AOC vs DAC: When to Use Active Optical Cables vs Direct-Attach Copper

Use a Cisco QSFP-100G DAC (passive direct-attach copper, QSFP-100G-CU series) for short, in-rack 100G links of 5 meters or less — it is the cheapest option and draws almost no power. Use a QSFP-100G AOC (active optical cable, QSFP-100G-AOC series) when you need to reach 7 to 30 meters across rows or between racks, where copper cannot carry 100G reliably. Rule of thumb: DAC for top-of-rack server-to-switch and switch-to-switch inside one rack; AOC for cross-rack and end-of-row aggregation. Beyond 30 meters, switch to pluggable optics (QSFP-100G-SR4, CWDM4, or LR4) over structured fiber.

DAC vs AOC at a glance

Both cables terminate in fixed QSFP28 connectors on each end and deliver a full 100G link, so from the switch's point of view they look identical — the same QSFP28 cage, the same 100GBASE link. The difference is the medium in the middle and how far it can carry the signal.

A DAC (direct-attach copper) is a passive twinax cable. There is no electronics and effectively no power draw; the two QSFP28 ends are just terminations on a shielded copper pair. Cisco's QSFP-100G-CU family conforms to 100GBASE-CR4 / IEEE 802.3bj and tops out at 5 meters. Because it is passive and copper, it is the cheapest way to make a 100G connection and it adds no heat to the rack.

An AOC (active optical cable) puts a small laser and receiver inside each QSFP28 end and runs multimode fiber between them. Cisco's QSFP-100G-AOC family is built on 100GBASE-SR4 optics (850 nm, MMF) and reaches up to 30 meters, drawing roughly 3.5 W per end. The fiber core is thinner and far lighter than copper twinax, which matters once you are bundling dozens of cables through a rack.

In short: DAC is the short-reach, lowest-cost, zero-power choice; AOC trades a little cost and power for several times the reach and much lighter, more flexible cabling.

Reach, cost, power, and weight compared

The table below summarizes the practical trade-offs for the Cisco QSFP-100G DAC and AOC families. These are the figures that actually drive a buying decision — reach first, then cost, power, and cable bulk.

Always confirm exact length availability and current pricing at order time, since Cisco adjusts both per release and the right cable depends on your physical rack layout.

Top-of-rack: where DAC wins

The classic top-of-rack (ToR) design puts a switch — commonly a Cisco Nexus 9300 such as the N9K-C93180YC-FX or N9K-C93240YC-FX2 running NX-OS — at the top of each server rack, with the servers cabled straight up to it. Every one of those links is short, usually well under 3 meters. This is exactly where a passive DAC belongs.

For ToR you get three things from copper that you do not need to pay for with optics:

• Lowest cost per link. A QSFP-100G-CU is the cheapest 100G connection Cisco sells. Across a rack of 40-plus servers, the savings versus AOC or optics add up quickly.
• No added power or heat. Passive twinax draws essentially nothing, so it does not contribute to the rack's thermal budget — unlike AOCs, which add ~3.5 W per end, or pluggable optics.
• No optics to fail. With no laser in the cable, there is simply less to go wrong on a short run.

Use DAC for server-to-ToR uplinks and for switch-to-switch links inside the same rack (for example, a Nexus 9300 to an adjacent leaf). The only real constraints are length and bulk: stay within 5 meters, and remember that thick twinax is stiff and heavy, so leave room for bend radius and airflow when a rack is densely cabled.

Cross-rack and end-of-row: where AOC wins

Once a link leaves the rack, copper runs out of reach. A passive QSFP-100G DAC stops at 5 meters, which is fine inside a cabinet but not enough to reach the next rack over, an end-of-row (EoR) aggregation switch, or a spine a few cabinets away. That is the AOC's job.

QSFP-100G-AOC cables span 7, 10, 15, 20, 25, and 30 meters — lengths a DAC simply cannot offer. Beyond reach, the active optical cable wins on two physical properties that matter at scale:

• Weight and bend radius. Fiber is dramatically thinner and lighter than 100G twinax. When you are routing dozens of cross-rack links through overhead trays or under-floor channels, AOCs are far easier to manage and put less strain on the QSFP28 ports.
• Airflow. Slim cables block less air than dense bundles of stiff copper, which helps in tightly packed rows.

The trade-off is cost and power: an AOC costs more than a DAC of similar length and adds roughly 3.5 W per end. That is a fair price for reaching across the room. Use AOC for leaf-to-spine links a few racks apart, EoR server connections, and any 100G run between 7 and 30 meters where pulling structured fiber and plugging in transceivers would be overkill.

When to skip both and use optics

AOCs and DACs are fixed-length, point-to-point cables — both ends are permanently attached. That is perfect inside and across rows, but it stops being practical once distances grow or you need structured cabling you can patch and reconfigure. At that point, switch to pluggable QSFP28 optics over your own fiber plant.

The usual escalation, by distance:

• 0–5 m, in-rack: QSFP-100G-CU passive DAC.
• 7–30 m, cross-rack / EoR: QSFP-100G-AOC active optical cable.
• Up to ~70–100 m on OM3/OM4 multimode: QSFP-100G-SR4 transceivers with MPO fiber.
• Up to 2 km on single-mode: QSFP-100G-CWDM4.
• Up to 10 km on single-mode: QSFP-100G-LR4.

The deciding factors are distance, whether you need a structured patch field you can re-route, and whether the link crosses between buildings or floors. If the run is fixed and short, a cable beats optics on cost and simplicity. If it is long, or you want the flexibility to repatch, optics over your own fiber is the right call. Make sure the QSFP28 port supports the optic you choose — most modern Nexus 9000 (NX-OS) and Catalyst 9500 high-speed (IOS-XE) QSFP28 ports do, but verify against the platform's compatibility matrix before you order.

If you want help matching cables and optics to a specific Nexus or Catalyst build, browse the full Cisco optics and transceivers catalog or talk to us about the switch platform you are wiring up.

How to choose for your rack

A simple decision flow covers almost every 100G QSFP28 link:

• Is the link inside one rack and 5 m or less? Use a QSFP-100G-CU passive DAC. It is the cheapest, coolest, and simplest option.
• Does it leave the rack but stay within 30 m? Use a QSFP-100G-AOC. You get the reach and lighter, more manageable cabling.
• Is it longer than 30 m, or do you need a structured, re-patchable fiber plant? Use pluggable optics (SR4, CWDM4, or LR4) over your own fiber.

Two practical notes. First, leave headroom: buy the length you need plus slack for routing, but do not over-buy AOC reach you will never use — a 30 m cable for a 4 m run is wasted money and harder to dress neatly. Second, standardize. Picking one DAC length for ToR and one or two AOC lengths for cross-rack keeps spares simple and procurement clean.

Both cable types interoperate across Cisco QSFP28 platforms — the same DAC or AOC works on a Nexus 9300 leaf and a data center switch spine — so you are choosing on reach and rack layout, not on which switch sits at each end.