400G AOC vs. DAC: Which Is Better for Short Router-to-Router Links?
For two routers in the same rack, compare the cable route, power per end, port support and replacement access before choosing a 400G DAC or AOC.
AXONODE / NETWORK DEPLOYMENT
For two routers in the same rack, the useful comparison starts with the cable route, the power at each end and how the connection will be maintained.
Imagine two routers mounted close together in a cabinet. Their 400G ports appear to be about 30 centimeters apart, so the purchasing note says “0.3m AOC.” It sounds specific enough to order.
There are still two decisions hidden in that line: whether 0.3m is the right installed length, and whether the connection needs an optical cable at all.
For a short, uncomplicated router-to-router connection, we would put a supported passive DAC on the initial shortlist. AOC deserves consideration when the route is longer, the copper assembly is awkward to manage, or the installation benefits from a lighter cable. That is a starting point for comparison, not a rule that applies to every rack.
This article concerns a straight-through 400GbE link between two ports. A 400G-to-4×100G breakout or a connection between separate sites is a different design problem.
The 30cm gap is not the cable length
Measure the route the cable will actually take. It may need to leave the port straight, turn toward a cable manager, pass around another assembly and return to the second router. Allow access to the release tabs and enough room to remove a module without pulling on its neighbor.
There is a reason to check the cable drawing rather than measure only the free space between ports. NVIDIA distinguishes the bend radius along a cable from the larger radius that may be required near its connector. It also notes that suppliers can define assembly length differently: some measurements include the portion of the connector inserted into the equipment, while others exclude it. [1]
A catalog length is not the same thing as usable routing distance.
Arista's published portfolio, for example, lists 400G QSFP-DD passive copper cables at 1m, 2m and 2.5m, and 400G AOCs at selected lengths from 1m to 30m. These are examples from particular cable families, not universal minimum or maximum lengths. [2]
For a request specifying 0.3m, we would first ask for the port positions and intended routing. A standard assembly may be more practical than a custom one, but neither should be selected before checking bend clearance and slack storage.
Start with passive copper for the simple connection
A passive direct-attach copper cable carries the high-speed electrical signal over twinax copper. Unlike an active cable, it does not use signal-conditioning electronics in the cable to boost or regenerate that signal. It relies on the endpoint hardware to support the electrical channel. Juniper describes passive DAC as a lower-cost option for very short interconnects. [3]
That makes it a reasonable first candidate when the two routers are close together and the cable has a clean route. There is little value in paying for optical reach that the installation will never use.
Still, “DAC” is not a complete specification. Active copper assemblies add signal-conditioning components, and their reach, power and compatibility need separate consideration. NVIDIA's linear active copper documentation illustrates that distinction. An active electrical cable should not inherit the assumptions made for a passive DAC. [4]
There is also no extra bandwidth prize for choosing fiber: if both qualified assemblies establish the intended 400GbE link, AOC does not make that interface faster. The purchasing decision is about whether the assembly suits the installation, not whether “optical” sounds like the higher-performance choice.
AOC earns its place when the route becomes awkward
AOC converts the signal to light inside the cable ends and carries it over fiber. Its external connection to each host is still electrical. The fiber cable is typically lighter than a comparable copper assembly, which can make routing easier in a crowded rack. [3]
That can be useful even over a short distance. A cable that reaches comfortably but blocks access to neighboring ports is not a particularly good installation. For a several-meter route between racks, we would also compare supported AOC options rather than assume a passive copper assembly will cover the distance.
The maintenance trade-off deserves attention. An AOC is a fixed assembly: the optical ends and fiber are permanently attached. It is not a pair of removable transceivers with an interchangeable patch cord. [4]
Consider how a replacement would be installed. Could a technician access both ends without disturbing other services? Is there room to route a complete spare through the cable manager? If the connection must pass through optical patch panels, separate pluggable optics and connectorized fiber are generally the more appropriate arrangement.
For a single, accessible router pair, a fixed AOC may be perfectly convenient. For a route that is difficult to reach, replacement effort belongs in the comparison alongside the purchase price.
Read the power figure per end
A thin cable can make the front of a rack less crowded. It does not tell you how much heat the cable ends add to the ports.
Cisco's 400G QSFP-DD datasheet gives a useful, product-specific example: [5]
| Cable family | Published maximum power per end | Sum for both ends |
|---|---|---|
| QDD-400-CUxM passive copper | 1.5W | 3W |
| QDD-400-AOCxM active optical | 12W | 24W |
These are published maximum figures for those Cisco families, not measured operating consumption, Axonode specifications or a universal DAC-versus-AOC ratio. The two-end totals are arithmetic sums; they do not include the routers' own power consumption. The table also does not establish compatibility with an Arista platform.
It does show why “passive means exactly zero watts” and “AOC is always cooler” are poor purchasing shortcuts. Ask for the actual proposed part number's rating, and check whether it is quoted per end or per complete assembly. [5]
For a remote cabinet, we would evaluate endpoint power together with cable routing and the host's cooling limits. A cable choice should not depend on jacket thickness alone. The related article on 400G optics in outdoor cabinets covers the broader thermal questions.
The same speed on the label is not enough
Before comparing prices, identify the exact port at each end. Arista offers 400G connectivity in both QSFP-DD and OSFP form factors, and its EOS documentation shows that port profiles can change the number of interfaces and their operating speeds. “Two Arista routers with 400G ports” leaves too much unspecified. [2][6]
A useful request includes both hardware models, the selected ports, software releases, connector form factors and the intended native 400GbE mode. For mixed-vendor equipment, check both endpoints rather than treating acceptance at one end as proof for the other.
We would also verify the cable's supported electrical interface, port power allowance and required forward error correction settings. Host recognition is only one part of qualification; the link must establish and carry traffic in the intended configuration.
A short cable is not a reason to disable FEC. Cisco documents the host-side RS(544,514) requirement for its PAM4-based 400G QSFP-DD optics. The applicable setting for a particular cable and platform should come from its support documentation, not an assumption that one meter is too short for errors. [7]
Give the candidate a proper trial
Before buying a batch, we would test one proposed assembly on the intended pair of ports. Record the part number, cable length, endpoint software and configuration. Route it as it will be installed, including the cable manager and normal cabinet-door clearance.
Then check link establishment and traffic forwarding, and inspect the available interface, CRC and FEC statistics. A nonzero corrected-FEC counter alone is not proof that a cable is defective: FEC exists to correct errors. Equally, correction activity should not be ignored simply because the link stays up. [7]
For acceptance, compare error rates and trends with the platform's guidance. Investigate uncorrectable errors, packet loss and link flaps. In a controlled test or maintenance window, verify that the connection recovers after a restart. Where available, record the cable-end temperature readings alongside the operating conditions.
There is a physical check too: can the door close without pressing on the cable, can adjacent modules be removed, and could the same route accept a replacement without improvisation?
For a straightforward in-rack link, our first comparison would usually favor a supported passive DAC on cost and endpoint power. We would choose AOC when its reach or routing advantages justify the additional active electronics. If patch panels and separately replaceable components are priorities, we would examine discrete optics and fiber instead.
The most useful quote is therefore a proposed assembly for a known pair of ports and a measured route—not simply the cheapest item labeled “400G.”
For an Axonode cable review, send the two device models, selected port types and the routed cable length. Include software versions and a photo of the proposed route when available. We can then review suitable DAC or AOC candidates and identify what still needs validation before an order.
Discuss the connection with Axonode · jenny@axonodesolution.com
