400G DR4 vs FR4 vs LR4: Which QSFP-DD Reach Code Do You Need?

Two 400G QSFP-DD ports on the same switch can need completely different modules. One runs 500 m inside a GPU pod, the next runs 10 km to a campus building, and a third reaches a data center across town. The cages look identical, but the reach code behind each link decides which fiber you pull, how many strands you terminate, how much power the line card carries, and which FEC mode both ends negotiate. Ordering by speed alone is a common purchasing error: a module specified only as "400G" can land as DR4, FR4, LR4 or ZR, and those families are not interchangeable on the same span.

400G DR4 vs FR4 vs LR4 vs ZR at a glance

400G DR4 at 500 m: the shortest and lowest cost lane

DR4 is the workhorse of the AI pod: four 100G PAM4 lanes in parallel at 1310 nm, staying inside a rack row or between two rows in the same hall. Because the module stays short, it uses the least power and the simplest thermal budget of the 400G family, which matters when 32 or 64 ports sit on one line card. The 400G QSFP-DD DR4 500 m module carries DDM and DOM telemetry, so receive power and temperature can be read from the switch before a link starts to flap.

400G FR4 at 2 km: duplex fiber when breakout is not an option

FR4 puts four CWDM lanes onto a standard duplex single mode pair. That removes breakout cabling and multi-fiber connector cleaning from the project, which is why FR4 dominates leaf-spine fabrics and campus aggregation. If your campus fiber is already duplex LC patched, FR4 is usually the cheapest path to 400G because nothing in the patch panel changes. Review the 400G QSFP-DD FR4 2 km module for distances between 500 m and 2 km, and step up to a parallel DR4+ or XDR4 part only if the patch plan already supports multi-fiber trunks.

400G LR4 at 10 km: campus, metro access and operator handoff

LR4 keeps the duplex pair but pushes reach to 10 km using four LAN-WDM lanes near 1310 nm. It is the code campuses and operators use where a 400G fabric meets a metro ring or a handover point. Power draw and thermal load run higher than a 500 m DR4, so cabinet airflow and per-port power budget should be checked before the bill of materials is frozen. The 400G QSFP-DD LR4 10 km module is the part to quote when the same 400G port serves a building 3 km away and a handoff 8 km away without changing the optics family mid-project.

Beyond 10 km: 400G ZR and ZR+ coherent

Past 10 km, direct detect reach codes stop being a design choice. Coherent 400G ZR uses DSP based modulation and a stronger FEC to reach roughly 120 km on unamplified duplex fiber, and ZR+ extends that to 450 km on amplified spans. Coherent modules cost more and draw more power, but they replace a full transport chassis on many metro DCI routes. Start with the 400G QSFP-DD ZR 120 km coherent module and confirm span loss and amplifier placement with our engineering team before ordering.

What else changes when the reach code changes

• FEC. 400G direct detect links negotiate RS(544,514) KP4 forward error correction at both ends. A FEC mismatch is a link that never comes up, even with two healthy modules.
• Power class. Reach codes sit in different power bands, from the leanest 500 m DR4 to coherent ZR+. Confirm the figure on the datasheet before finalizing rack power and airflow.
• Patching and breakout. Duplex codes keep the patch panel simple; parallel codes need multi-fiber trunks and end face cleaning. Port breakout to 4 x 100G depends on the switch platform, not the optic alone.
• Coding. Modules are coded to match the host switch or router, so tell us the platform and firmware generation before shipment.

How APEX supports a 400G QSFP-DD rollout

APEX builds and supplies 400G QSFP-DD modules for AI data center and telecom networks, with the reach codes above held as standard part numbers rather than special orders. Every unit is coded to the target host platform and tested for optical power, BER and DDM readback before shipment.

• One part number per reach code, from APX-QSFP-DD-400G-500m-DR4 through APX-QSFP-DD-400G-450km-ZR+.
• Coding and compatibility checks for the host switch or router, including mixed 100G and 400G ports in one chassis.
• Reach and link budget review: send the span, fiber type and patch plan and we confirm which code fits.

FAQ

Can 400G FR4 or LR4 run over multimode fiber?

No. FR4, LR4 and ZR are single mode codes. For short runs inside a hall, choose a DR4 class module and confirm the fiber type on the datasheet before you pull cable.

Can I mix DR4, FR4 and LR4 on the same switch?

Yes. They share the QSFP-DD cage, so the mix is a per-port decision. What cannot be mixed is one link: both ends of a span need the same reach code and the same FEC mode.

Do I need a different module if the run is 1.8 km instead of 2 km?

No. FR4 at 2 km and the 2 km DR4+ and XDR4 parts all cover that span. The deciding factor is your cabling plan, not the extra 200 m.

Why does a 400G LR4 draw more power than a 400G DR4?

Longer reach needs more optical output and a stronger DSP path, so the module sits in a higher power class. The exact figure for each part number is on its datasheet.

Is 400G ZR worth it for a 15 km metro link?

Usually not. Coherent ZR is built for spans well past 10 km, so LR4 with proper margin is normally the honest answer at 15 km. Send the measured span loss and we will confirm.

Next step

Send your bill of materials with the details that decide the reach code:

• Switch or router platform, port count and firmware generation for coding
• Measured distance for each link group, not the nominal site distance
• Fiber type and patch panel plan for each span

We map every link to a reach code, confirm coding, and return pricing and lead time for the APX parts that fit. Ask for a quote with your BOM, distances and platform, and we come back with a coded configuration ready to drop into the switch.