800G/400G/200G DAC Breakout Guide: CR1, CR2, CR4, CR8 & Mixed-Speed Applications

1. What Is a DAC Breakout Cable?

With the rapid development of AI data centers, cloud computing, and high-speed Ethernet, more and more switches now provide 400G and 800G high-speed ports, while the server side still widely deploys 100G, 50G, and even 25G networks.

To provide compatibility between devices operating at different speeds, DAC (Direct Attach Copper) products have evolved beyond standard direct-attach cables to include Breakout DAC cables.800G 400G 200G 100G AOC/DAC/AEC/ACC Cable丨C-LIGHT

The primary purpose of a Breakout DAC is to split one high-speed port into multiple lower-speed ports, maximizing port utilization.

For example:

• 800G → 8 × 100G

• 400G → 4 × 100G

• 200G → 2 × 100G

• 100G → 4 × 25G

This solution is widely used in Spine-Leaf architectures, GPU server connectivity, and storage networks.

2. What Do CR1, CR2, CR4, and CR8 Mean?

In DAC products, you will often see designations such as CR1, CR2, CR4, and CR8.

Here, CR stands for Copper Reach, referring to the physical copper cable lanes, while the number indicates the number of electrical lanes.

Therefore:

• 200G QSFP56 DAC typically belongs to CR4, while 200G QSFP112 DAC belongs to CR2.

• 400G DAC typically belongs to CR4.

• 800G DAC typically belongs to CR8.

The number of lanes determines the available breakout configurations.

3. Why Does the Industry Mainly Adopt Rate-Matched Breakout?

Today, the vast majority of DAC breakout solutions follow one fundamental principle:

The total bandwidth remains unchanged, while only the lane resources are redistributed.

For example:

The main advantages include:

• No data rate conversion

• No retimer required

• No protocol conversion

• PHY only remaps the electrical lanes

• Lowest latency

• Lowest cost

• Lowest power consumption

For these reasons, this is also the breakout method recommended by IEEE Ethernet standards.

4. Are There Non-Rate-Matched Breakout Solutions?

The answer is yes, but they are extremely uncommon.

In the industry, what is often referred to as a "non-rate-matched" breakout is generally not a true data rate conversion. Instead, it refers to differences in connector form factors and internal lane mapping.

For example, the Arista Active DAC solution that converts 400G QSFP-DD to 4 × QSFP28 DAC is a typical example.

Many engineers initially wonder how a QSFP-DD interface can be split into QSFP28 connectors.

In reality, the QSFP28 PCB design integrates a dedicated Gearbox (essentially a DSP processor), which converts the incoming 2 × 50G PAM4 signals from the QSFP-DD side into 4 × 25G NRZ signals at the QSFP28 interface. This enables signal conversion while maintaining the optical port protocol compatibility. The overall bandwidth remains 400G = 4 × 100G.

Therefore, it is still considered a Rate-Matched Breakout.

It is not a true bandwidth conversion such as:

• 400G → 4 × 40G

• 400G → 4 × 25G

• 400G → 2 × 200G + 4 × 25G

These would represent genuine rate conversion rather than standard breakout.

5. C-LIGHT DAC Breakout Solutions

To support different network architectures, C-LIGHT provides a complete portfolio of DAC Breakout products, 

800G/400G/200G/100G/50G/40G/25G/10G DAC Cable丨C-LIGHT

including:

• 100G QSFP28 → 4 × 25G SFP28

• 200G QSFP56 → 2 × 100G QSFP56 or 4 × 50G SFP56

• 400G QSFP56-DD → 4 × 100G QSFP56 or 2 × 200G QSFP56

• 800G OSFP112 / QSFP-DD112 → 8 × 100G QSFP112 or 4 × 200G QSFP112

• 400G OSFP56 → 2 × 200G QSFP56

• Custom vendor coding EEPROM and ID support

• Custom lane mapping, cable colors (black, brown, red, orange, yellow, green, purple, gray, pink, white), and pull-tab colors

• Multiple wire gauges (26 AWG, 28 AWG, 30 AWG)

• Custom cable lengths and label colors

These solutions meet the deployment requirements of AI data centers, cloud computing, high-performance storage, and enterprise networks.

6. Conclusion

The core design principle of DAC breakout is always based on lane resource redistribution.

CR1, CR2, CR4, and CR8 not only represent the number of copper lanes but also determine how many lower-speed ports can be split from a high-speed interface. Most industry-standard breakout solutions follow the principle of maintaining the same total bandwidth, providing the lowest latency, lowest power consumption, and the best compatibility.

Frequently Asked Questions (FAQ)

Q1: What do CR1, CR2, CR4, and CR8 represent?

A: They represent the number of Copper Reach (CR) lanes in a DAC cable—1, 2, 4, and 8 high-speed electrical lanes respectively. These lane configurations determine the breakout capability of the cable.

Q2: Do all DAC cables support breakout?

A: No. Breakout is only possible when the switch ASIC, interface type, and DAC cable are all designed to support breakout functionality.

Q3: What is the biggest difference between an Arista Breakout DAC and a standard DAC?

A: The primary differences lie in the vendor EEPROM coding, lane mapping, and compatibility optimizations for the Arista EOS platform, rather than the transmission speed itself.

Q4: Are there truly non-rate-matched DAC breakout solutions?

A: Standard passive DAC cables generally do not support true rate conversion. If a 400G interface needs to be converted into multiple interfaces with different speeds (such as mixed 100G and 25G outputs), an active cable (ACC/AEC) with Retimer or Gearbox functionality, or a switching ASIC, is typically required instead of a standard passive DAC.