{"id":12079,"date":"2026-04-23T16:01:58","date_gmt":"2026-04-23T08:01:58","guid":{"rendered":"https:\/\/ascentoptics.com\/blog\/?p=12079"},"modified":"2026-04-23T16:01:58","modified_gmt":"2026-04-23T08:01:58","slug":"qsfp28-breakout-cable","status":"publish","type":"post","link":"https:\/\/ascentoptics.com\/blog\/qsfp28-breakout-cable\/","title":{"rendered":"QSFP28 Breakout Cable: Complete 100G-to-25G Guide"},"content":{"rendered":"

David Park ordered 400 pieces of QSFP28-to-4\u00d7SFP28 breakout DACs for a new leaf switch deployment in Seoul. The cables looked good and seated properly. However, the team soon discovered that only half of the switch ports actually supported breakout mode. The remaining ports could only run at 100G. With 200 cables left unused in the corner, the server cutover was delayed by a full week and the project went $30,000 over budget.<\/p>\n

This is a classic example of the hidden challenges in QSFP28 breakout cable deployments. The cable itself is only one piece of the puzzle \u2014 switch port support, distance, power budget, and signaling compatibility all play critical roles in successful operation.<\/p>\n

This guide will help you select the right breakout cable for your 100G-to-25G architecture, verify switch compatibility, and avoid the common mistakes that delay deployments.<\/p>\n

Need help selecting breakout cables?<\/strong>\u00a0Explore Ascent Optics’ QSFP28 connectivity solutions<\/a>\u00a0or\u00a0contact our engineers<\/a>\u00a0for a free compatibility review.<\/p>\n

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What Is a QSFP28 Breakout Cable?<\/strong><\/h2>\n

A QSFP28 breakout cable splits a single 100G QSFP28 port into four separate 25G connections. This architecture divides the 100G signal into four independent 25G lanes, allowing one leaf switch port to connect to four 25G server ports.<\/p>\n

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\"4\u00d725G<\/p>\n

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How Breakout Architecture Works (4\u00d725G)<\/strong><\/h3>\n

A 100G rate of transmission is transmitted via four 25G NRZ lanes in the QSFP28 modules. In a breakout cable, these four lanes are physically allocated to four individual SFP28 connectors. Each SFP28 is fed from one lane of 25G to a server NIC or another switch port.<\/p>\n

This design has become the standard for modern data centers because it maximizes port utilization. A single 48-port QSFP28 leaf switch that fully supports breakout can connect up to 192 \u00d7 25G servers.<\/p>\n

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QSFP28 Breakout vs Standard QSFP28 Transceivers<\/strong><\/h3>\n

Breakout cables are not transceivers. They provide a direct-attach copper or optical assembly between one QSFP28 port and four SFP28 ports for short-reach links. For longer distances or structured cabling, the alternative is to use separate QSFP28 and SFP28 transceivers over a fiber plant.<\/p>\n

For a broader introduction to 100G connectivity, see our\u00a0QSFP28 transceiver guide<\/a>.<\/p>\n

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QSFP28 Breakout Cable Types Explained<\/strong><\/h2>\n

Not all breakout cables are built the same. The right type depends on distance, power budget, and physical routing constraints.<\/p>\n

Passive Direct Attach Copper (DAC)<\/strong><\/h3>\n

Passive DAC cables<\/a> use copper traces without signal amplification. They are the lowest-power and most cost-effective option. Typical lengths range from 1 to 3 meters (some manufacturers offer up to 5 meters).\u00a0Passive DACs are ideal for servers located in the same rack as the leaf switch or in adjacent racks.<\/p>\n

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Active Direct Attach Copper (Active DAC)<\/strong><\/h3>\n

Active DACs include electronic signal conditioning inside the QSFP28 connector, allowing reaches of 5 to 10 meters. They consume slightly more power than passive DACs but remain significantly cheaper than AOCs.\u00a0Active DACs work the best for racks across an aisle or next to racks where passive copper fails to maintain signal integrity.<\/p>\n

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Active Optical Cable (AOC)<\/strong><\/h3>\n

AOC breakout cables<\/a> use multimode fiber with integrated optical transceivers at both ends. They can reach 30 to 100 meters depending on fiber grade and design. Although more expensive and power-intensive than DACs, AOCs solve distance and electromagnetic interference issues that copper cannot handle.<\/p>\n

AOCs are commonly used when servers are more than 10 meters from the leaf switch or when copper becomes unreliable due to EMI.<\/p>\n

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Breakout Optical Fiber Cables (MTP to LC)<\/strong><\/h3>\n

In a structured cabling situation, you might install a QSFP28 SR4 transceiver<\/a> on the switch side and break this out into four SFP28 SR transceivers using a MTP-to-4\u00d7LC breakout harness on the server side. This allows one to use traditional optical transceivers instead of direct-attach cables and is the preferable mode of action when already having fixed structural cabling in place.<\/p>\n

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\"DAC<\/p>\n

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DAC vs AOC: How to Choose<\/strong><\/h2>\n

The DAC versus AOC decision is usually the first technical choice in a breakout deployment.<\/p>\n

Distance Comparison<\/b><\/strong><\/h3>\n\n\n\n\n\n\n
Cable Type<\/b><\/strong><\/td>\nTypical Reach<\/b><\/strong><\/td>\n<\/tr>\n
Passive DAC<\/td>\n1\u20133 meters<\/td>\n<\/tr>\n
Active DAC<\/td>\n5\u201310 meters<\/td>\n<\/tr>\n
AOC<\/td>\n30\u2013100 meters<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

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Power and Thermal Comparison<\/strong><\/h3>\n

Passive DACs consume less than 0.5 W per end. Active DACs typically draw 1.5\u20132.5 W, while AOCs range from 2\u20134 W. The lower power of DACs makes them popular in power-constrained designs.<\/p>\n

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Cost Comparison<\/strong><\/h3>\n

Passive DACs are the cheapest per link. Active DACs are typically 20\u201340% more expensive than passive versions. For the same breakout configuration, AOCs can be 3\u00d7 to 5\u00d7 more expensive than passive DACs.<\/p>\n

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When to Choose DAC<\/strong><\/h3>\n

Choose a DAC when:<\/p>\n