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By Topnet Engineering Team | Updated July 2026
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400G remains the volume workhorse of data center networking: it powers spine and leaf tiers in Ethernet fabrics, connects storage and compute clusters, and serves as the breakout tier under new 800G deployments. This guide covers the three ways to build a 400G link - optical transceivers, DAC (direct attach copper) and AOC (active optical cable) - and how to choose between QSFP-DD and OSFP form factors, reach classes, and cable types for the lowest cost per link.
Both form factors deliver 400G over 8x50G PAM4 (or 4x100G in newer silicon). QSFP-DD dominates Ethernet switching because its cage is backward compatible with QSFP28/QSFP+, protecting existing investment. OSFP appears mainly in AI-cluster gear and offers more thermal headroom. If your switches are standard Ethernet data center models, QSFP-DD is almost always the answer at 400G; check each device's port type before ordering and never assume both link ends match.
| Type | Fiber / Connector | Reach | Typical use |
| 400G SR8 | MMF OM4, MPO-16 | 100m | In-row switch links, lowest optics cost |
| 400G SR4 | MMF OM4, MPO-12 | 100m | Newer 4x100G short links |
| 400G DR4 | SMF, MPO-12 APC | 500m | Leaf-spine inside the building; breaks out to 4x100G-DR |
| 400G FR4 | SMF, duplex LC | 2km | Campus / cross-building, duplex cabling saves fiber |
| 400G LR4 | SMF, duplex LC | 10km | Data center interconnect and metro |
DR4 is the de facto standard for leaf-spine because a single 400G-DR4 port can also break out into four 100G-DR links - very useful during 100G-to-400G migration. FR4 and LR4 run over ordinary duplex LC single-mode, so no MPO trunk changes are needed for longer links.
| Factor | DAC (passive copper) | AOC (active optical cable) | 2x Transceivers + fiber |
| Max reach | ~3m passive (5m active) | up to 100m | 100m to 10km+ |
| Power per end | ~0W (passive) | ~4-5W | ~8-12W |
| Cost per link | Lowest | Medium | Highest, but reusable fiber plant |
| Weight / bend | Heavy, stiff | Light, flexible | Light structured cabling |
| Best for | In-rack ToR to server | Row-level switch links | Anything beyond the row, structured plants |
A simple rule that fits 90% of deployments: DAC inside the rack, AOC inside the row, transceivers beyond the row. Passive 400G QSFP-DD DAC costs a fraction of an optical link and consumes virtually no power - multiply that saving by hundreds of server links and the numbers are significant. AOC wins where copper is too short or too bulky but a structured fiber plant is not justified. Once links leave the row or need patch-panel flexibility, transceivers plus trunk cabling are the scalable answer.
1) Port type on both ends - QSFP-DD vs OSFP vs QSFP112. 2) Breakout needs - 400G to 4x100G or 2x200G requires the right module type and switch support. 3) Fiber polarity - MPO-12 APC Type-B for DR4-class links. 4) DAC gauge and length - passive copper above 3m becomes unreliable; step up to AOC. 5) Sample first - validate in your own chassis before volume orders.
Yes - QSFP-DD cages are backward compatible with QSFP28 and QSFP+ modules, which run at their native speed in the new port.
DR4 uses four parallel 100G lanes over MPO-12 single-mode fiber for 500m and supports 4x100G breakout; FR4 multiplexes four wavelengths (CWDM4) onto duplex LC single-mode for 2km but cannot break out.
Passive DACs are largely vendor-agnostic electrically, but switches validate the EEPROM coding. Order DACs coded for your platforms, or dual-coded ends for mixed-vendor links - specify both device models when ordering.
Choose AOC for fixed point-to-point links up to 100m where you will not re-patch: it costs less than two SR modules plus fiber and is factory-terminated. Choose transceivers where you need patch-panel flexibility or longer reach.
Topnet System offers samples under a refundable-deposit, 30-day testing program - see the Free Sample & Testing Policy. Test reports are available on request.
Topnet System (ISO certified since 2008) manufactures the complete 400G family - QSFP-DD and OSFP transceivers (SR8/DR4/FR4/LR4), passive DAC assemblies, and AOC in standard and breakout configurations - alongside our 800G portfolio for your next upgrade tier. Email sales@topnetsystem.com for datasheets, samples and volume pricing.