400G QSFP-DD to 4x 100G QSFP28 AOC and DAC

A 400G QSFP-DD port can be split into four independent 100G QSFP28 links using a breakout cable. This allows a single high-density 400G switch port to serve four 100G endpoints—servers, NICs, storage nodes, or leaf switches—without requiring a separate 400G transceiver at each endpoint. The breakout cable carries eight 50G PAM4 electrical lanes at the 400G end and maps them to four 25G NRZ lanes at each of the four 100G ends.

Two cable technologies dominate this configuration: Direct Attach Copper (DAC) and Active Optical Cable (AOC). DAC uses twinax copper and reaches up to 3 meters with near-zero power. AOC uses multimode fiber and reaches up to 100 meters with active optical-to-electrical conversion at each end. Between them sits a third option, Active Electrical Cable (AEC), which uses retimer-based copper to cover the 3–5 meter range that passive DAC cannot span.

This article covers the specifications of each breakout type, compares their reach, power, and cost characteristics, and examines alternative architectures for 400G-to-4x100G connectivity, including 400G SR4 to 4x100G SR1 optical module breakout.

1. Product Overview: 400G QSFP-DD to 4x100G QSFP28 Breakout

The breakout cable has one QSFP-DD connector at the 400G end and four QSFP28 connectors at the 100G end. The QSFP-DD end uses eight electrical lanes running at 53.125 Gbps PAM4 per lane. The four QSFP28 ends each use four electrical lanes running at 25.78125 Gbps NRZ per lane, for a total of sixteen lanes across the four 100G links. The cable contains a built-in gearbox that converts the eight PAM4 lanes to sixteen NRZ lanes.

Breakout cables are deployed in leaf-spine architectures where the spine switch provides 400G ports and the leaf switches or servers use 100G ports. A single 400G QSFP-DD port on the spine can connect to four 100G leaf switches or four 100G NICs in a server, reducing switch port consumption by 75% compared to using individual 100G ports for each connection. This design increases single-port bandwidth utilization by 300% and conserves switch port resources.

1.1 Breakout Cable Types

2. DAC Breakout: 400G QSFP-DD to 4x100G QSFP28

A passive DAC breakout uses twinax copper with no active signal conditioning. It provides the lowest cost, lowest power, and lowest latency solution for breakout connectivity. The cable is constructed with solid copper conductors and is available in 30 AWG and 27 AWG configurations.

2.1 Key Specifications

The passive DAC breakout is the lowest-cost option for 400G-to-4x100G connectivity. Its near-zero power consumption and low latency make it attractive for same-rack and adjacent-rack connections where the distance is under 3 meters. The downside is limited reach and cable stiffness. Passive DAC also demands tighter signal integrity from the host ASIC, because there is no active signal conditioning to compensate for channel loss.

3. AOC Breakout: 400G QSFP-DD to 4x100G QSFP28

An AOC breakout integrates optical transceivers at both ends of a multimode fiber cable. The QSFP-DD end contains eight optical lanes running at 53.125 Gbps PAM4, and the four QSFP28 ends contain four optical lanes each running at 25.78125 Gbps NRZ. The cable is factory-terminated and tested, so there are no optical connectors to clean or align in the field.

3.1 Key Specifications

The AOC breakout extends the reach to 100 meters over OM4 multimode fiber, covering inter-rack and cross-row connections that DAC cannot reach. The optical transceivers at each end provide active signal conditioning, so the cable tolerates more channel loss than passive DAC. The trade-off is higher cost and higher power consumption.

4. AEC Breakout: The Middle Ground

Active Electrical Cable (AEC) fills the reach gap between passive DAC (3 m) and AOC (70–100 m). AEC uses copper conductors with integrated Retimer chips at the connector ends. The Retimers perform clock and data recovery, signal reshaping, and equalization, extending copper reach to 5 meters or more.

4.1 Key Specifications

AEC provides 25–50% lower power consumption than AOC at comparable distances and supports reaches that passive DAC cannot achieve. The Retimer-based signal conditioning ensures reliable signal recovery over the longer copper run. AEC is particularly popular in AI clusters where the 3–5 meter range between GPU nodes and Top-of-Rack switches must be covered without the cost and power of optical cables.

5. DAC vs AOC vs AEC: Comparison

6. Applications

The 400G QSFP-DD to 4x100G QSFP28 breakout cable is deployed in several data center scenarios.

6.1 Spine-to-Leaf Connectivity

In a leaf-spine architecture, the spine switch provides 400G ports and the leaf switches use 100G ports. A single 400G QSFP-DD port on the spine connects to four 100G leaf switches using one breakout cable. This reduces the number of spine switch ports required by 75% compared to using individual 100G ports for each leaf connection.

6.2 Server Fan-Out

In high-density server racks, a 400G switch port can connect to four 100G NICs using a breakout cable. This allows the switch to serve four servers from one port, simplifying cabling and reducing switch port consumption. The breakout cable is particularly useful in AI clusters where each GPU node requires high-bandwidth connectivity to the Top-of-Rack switch.

6.3 100G to 400G Migration

During the transition from 100G to 400G infrastructure, breakout cables allow operators to deploy 400G spine switches while continuing to use existing 100G leaf switches and NICs. The breakout cable bridges the two speed domains without replacing the 100G equipment. This enables a staged migration: deploy the 400G spine first, then upgrade the leaf and server layers as budget and demand allow.

6.4 Storage Network Upgrades

NVMe over Fabrics and distributed storage systems require high-bandwidth, low-latency connectivity. A 400G port broken out to four 100G links can connect four storage nodes or four storage controllers from a single switch port, simplifying the storage fabric and reducing port consumption.

7. Alternative Solutions for 400G-to-4x100G Connectivity

Beyond the breakout cable approach, several alternative architectures can achieve the same fan-out of one 400G port to four 100G endpoints. Each has its own trade-offs in reach, fiber count, cost, and deployment complexity.

7.1 400G SR4 to 4x100G SR1 Optical Module Breakout

This approach uses a 400G QSFP-DD SR4 optical module at the 400G end and four 100G QSFP28 SR1 or SFP112 SR1 optical modules at the 100G ends, connected through a fiber breakout harness. The 400G SR4 module transmits on four wavelengths over a multimode or single-mode fiber trunk, and the breakout harness splits the four channels into four independent 100G links.

GIGALIGHT’s 400G QSFP-DD SR4 to 4x100G QSFP28 SR1 solution uses single-lane 100G PAM4 modulation to achieve 100-meter transmission over OM4 multimode fiber with 20% lower power consumption than comparable alternatives. The solution reduces high-speed optoelectronic conversion units by 75% compared to traditional 400G full-optic module networks, lowering equipment procurement costs and minimizing fiber usage.

The module-based approach offers individual replaceability: if a 100G module fails, only that module is replaced, not the entire cable. This reduces sparing cost and downtime. It also allows different module types at each 100G endpoint—SR1, DR1, FR1, or LR1—to match the specific reach requirement of each link. The trade-off is higher initial cost and more complex fiber management.

7.2 400G DR4 to 4x100G DR1 Optical Module Breakout

For single-mode fiber deployments, a 400G QSFP-DD DR4 module can break out to four 100G QSFP28 DR1 links over eight single-mode fibers. The 400G DR4 module uses four lanes of 100G PAM4 over eight fibers (four transmit, four receive), and each lane maps to one 100G DR1 link with a reach of 500 meters. This approach extends the breakout reach beyond the 100-meter limit of multimode AOC.

The DR4-to-4xDR1 breakout is useful in large campus environments where the distance between the spine switch and the leaf switch or server exceeds the multimode fiber limit. It uses the same MPO-12 connector as the 400G DR4 module and a breakout harness to split the four lanes into four duplex LC links.

7.3 400G FR4/LR4 to 4x100G FR1/LR1 Optical Module Breakout

For metropolitan or long-reach deployments, a 400G QSFP-DD FR4 or LR4 module can break out to four 100G FR1 or LR1 links. The 400G FR4 module uses four CWDM wavelengths over a duplex LC fiber pair, and each wavelength maps to one 100G FR1 link with a 2 km reach. The 400G LR4 module uses four LAN-WDM wavelengths with a 10 km reach, and each wavelength maps to one 100G LR1 link.

Cisco supports QDD-4X100G-FR-S and QDD-4X100G-LR-S optics on 400G ports, which break out to 100G FR or LR links on the peer node. This approach allows a single 400G port to serve four 100G endpoints over long-reach single-mode fiber, with each endpoint at distances up to 10 km from the 400G switch.

7.4 Comparison of Breakout Cable vs Module Breakout

8. Selection Guidance

Selecting between breakout cable types and module breakout architectures depends on the distance, fiber plant, and operational requirements.

• Distance under 3 meters, cost-sensitive: Passive DAC breakout. Lowest cost and near-zero power, suitable for same-rack connections.

• Distance 3–5 meters: AEC breakout. Extends copper reach with retimer-based signal conditioning at moderate cost and power.

• Distance 5–100 meters: AOC breakout. Multimode fiber with integrated optics, covering inter-rack and cross-row connections.

• Distance 100–500 meters: 400G DR4 to 4x100G DR1 module breakout over single-mode fiber. Extends reach beyond AOC limits.

• Distance 500 meters to 10 kilometers: 400G FR4/LR4 to 4x100G FR1/LR1 module breakout. Long-reach single-mode with duplex LC fiber.

• Individual module replaceability required: Module breakout architecture. Modules can be replaced individually without replacing the entire cable.

9. Summary

The 400G QSFP-DD to 4x100G QSFP28 breakout cable splits a single 400G port into four independent 100G links, reducing switch port consumption by 75% and enabling staged migration from 100G to 400G infrastructure. Three cable technologies cover different distance ranges: passive DAC up to 3 meters, AEC up to 5 meters, and AOC up to 100 meters over multimode fiber.

Beyond breakout cables, optical module breakout architectures—400G SR4/DR4/FR4/LR4 to 4x100G SR1/DR1/FR1/LR1—extend the reach to 10 kilometers and provide individual module replaceability. These module-based approaches are suited for longer-reach deployments where AOC cannot span the distance, or where operational requirements favor replacing individual modules rather than entire cables.

The choice between breakout cable and module breakout depends on distance, fiber plant, and operational model. Breakout cables offer plug-and-play simplicity and lower cost for short-reach deployments under 100 meters. Module breakout offers flexibility, individual replaceability, and long-reach capability for campus and metro deployments.

10. Q&A