400G QSFP-DD to 100G Breakout – All Practical Options Explained

Posted by Fouad Mazhar on 2026 Jan 21st

As data centers and transport networks evolve to adopt 400G Ethernet, many architects face the challenge of breaking out 400G QSFP-DD ports into multiple 100G links, especially when connecting to existing 100G NRZ-based infrastructure.

This guide walks through all validated breakout options, explains key modulation differences, and proposes the right architecture depending on distance, compatibility, and budget.

PAM4 vs NRZ – The Modulation Wall

At the heart of breakout interoperability is the modulation mismatch between modern and legacy optics:

Modulation	Used In	Lane Speed	Compatibility
NRZ	Most 100G legacy optics (SR4, LR4, CWDM4, ER4L)	25G/lane	Incompatible with PAM4
PAM4	All 400G QSFP-DD optics (DR4, FR4, SR8) and 100G DR1/FR1	50G or 100G/lane	PAM4 family only

You cannot passively connect PAM4 to NRZ. You need either modulation conversion or optics with matching modulation on both sides.

Breakout Options Overview (2025)

Option	400G Module (QSFP-DD)	Breakout To	Cabling	Max Reach	Modulation	Status
1	400G-SRBD (BiDi)	4x 100G-SRBD/SR1.2	MPO → 4x LC	100m	PAM4	✅ Correct
2	400G-SR8	2x 100G-SR4	MPO-16 → 2x MPO-8	100m	NRZ (requires 200G)	⚠️ Bandwidth loss
3	400G-DR4	4x 100G-DR1	MPO → 4x LC	500m	PAM4	✅ Standard
4	400G-DR4+ / xDR4	4x 100G-FR1	MPO → 4x LC	2 km	PAM4	✅ Longer reach
5	400G-2FR4	2x 100G-CWDM4	Duplex LC	2 km	NRZ (200G mode)	⚠️ Bandwidth loss
6	400G-PLR4	4x 100G-LR1	MPO → 4x LC	10 km	PAM4	✅ Long-reach PAM4
7	4x SN DR4/FR4/PLR4	4x DR1/FR1/LR1	SN → LC (x4)	500m–10 km	PAM4	✅ Compact breakout
8	400G AOC (PAM4)	4x 100G QSFP28 NRZ	Fixed AOC cable	3–10 m	❌ No gearbox	⚠️ Not compatible
9	400G AOC w/ Gearbox	4x 100G NRZ	Fixed AOC	1–20 m	✅ PAM4→NRZ	✅ Works for NRZ
10	Passive DAC	2x 100G PAM4	Fixed Cable	1–3 m	PAM4	⚠️ Works w/ 50G NRZ or PAM4 only
11	Active Breakout Module	4x 100G CWDM4/LR4	SN or MPO32	Up to 10 km	PAM4→NRZ	✅ Limited reach
12	Muxponder-Based Breakout	4x 100G-ER4L	Any	Up to 30–40 km	✅ PAM4→NRZ	✅ Best for long reach

Key Use Case Scenarios

✅ Short Reach (≤10 km) – Modern PAM4 Networks

- Use direct DR4 → DR1 or PLR4 → LR1 breakout via MPO or SN cabling.
- All optics are PAM4 single lambda.
- No active conversion needed.

✅ Medium Reach (10–15 km) – Some Legacy Compatibility

- Active breakout modules that convert PAM4 to NRZ.
- Support CWDM4/LR4 optics.
- Max ~12 km depending on power budget.

✅ Long Reach (20–40 km) – Legacy NRZ (ER4L) Interop

- Use a muxponder to convert 400G PAM4 to 4x 100G NRZ.
- Each leg connects to ER4L optics rated for 30 km+.
- Clean electrical separation of each 100G stream.

Modulation Compatibility Summary

400G Side	100G Side	Passive Breakout	PAM4 to NRZ Needed?	Works?
DR4 (PAM4)	DR1/FR1/LR1 (PAM4)	✅ Yes	❌ No conversion needed	✅
DR4	LR4/CWDM4/ER4L (NRZ)	❌ No	✅ Needs active conversion	✅ via muxponder or AOE
SR8	SR4 (NRZ)	⚠️ Yes, but downrate to 200G	❌	⚠️ Bandwidth loss
Any	AOC/DAC (NRZ)	❌ AOC only works w/ gearbox	✅	✅ only with active cable

Summary: Match the Option to Your Application

Target Distance	100G Optics	Solution Type
<500m	DR1/SR1.2	Passive DR4 or SRBD
1–2 km	FR1/CWDM4	Passive PAM4 or active conversion
10 km	LR1/LR4	PLR4 passive or AOE
25–40 km	ER4L	Muxponder with NRZ output

Need help mapping your breakout options to your network design? We support:
- Mixed PAM4/NRZ interop
- Long-distance link planning
- Equipment recommendations
- QSFP-DD and 100G compatibility

Let’s make your 400G breakout plans robust and future-ready.