QSFP+ vs QSFP28 Cable

The short answer

A QSFP+ cable is normally used for 40GbE, with four electrical lanes running at approximately 10Gb/s each. A QSFP28 cable is normally used for 100GbE, with four lanes running at approximately 25Gb/s each.

Both use the same broad QSFP-family connector shape, so they can look interchangeable. They are not automatically interchangeable. The switch or adapter must support the intended Ethernet mode, lane rate, cable identification, breakout arrangement, and vendor compatibility.

For a reliable purchase, start with the equipment ports and protocol—not the plug shape:

1. Identify whether each end is intended for 40GbE or 100GbE.
2. Confirm the electrical lane speed and breakout mode.
3. Choose passive DAC, active DAC, AOC, or transceiver-based fiber.
4. Check the required distance and optical media.
5. Verify coding and compatibility with both host devices.

QSFP+ and QSFP28 at a glance

Characteristic	QSFP+	QSFP28
Common Ethernet use	40GbE	100GbE
Typical lane arrangement	4 × 10Gb/s	4 × 25Gb/s
Common direct-attach pairing	QSFP+ to QSFP+	QSFP28 to QSFP28
Common breakout pairing	40GbE to 4 × 10GbE	100GbE to 4 × 25GbE
Connector family	QSFP-family	QSFP-family
Cable media	Passive DAC, active DAC, AOC, or fiber with transceivers	Passive DAC, active DAC, AOC, or fiber with transceivers
Main compatibility risk	Mistaking identical connector geometry for 100GbE support	Assuming every QSFP28 port or cable supports every 40/100GbE mode

The table describes common deployments, not a universal rule for every switch, adapter, or cable. A port may support additional modes only when the manufacturer documents them.

Match the port type, protocol, and lane speed

The connector is only the mechanical interface

“QSFP+” and “QSFP28” identify related pluggable form factors and electrical generations. They do not describe one universal cable that works with every QSFP port.

A QSFP-family receptacle does not by itself tell you:

• Whether the port is configured for 40GbE or 100GbE
• Whether it supports breakout operation
• Which lane rate the host expects
• Whether the port requires a particular FEC mode
• Which cable EEPROM or vendor coding it accepts
• Whether the port is intended for DAC, AOC, or an optical transceiver

A QSFP28 port may support a 40GbE operating mode on some equipment, but that is a platform feature—not a guarantee created by the connector. Likewise, a QSFP+ port should not be assumed to carry 100GbE merely because a QSFP28-shaped assembly can be inserted.

Think in lanes, not just aggregate speed

The common lane calculations are:

4 lanes × 10Gb/s = 40Gb/s line-rate class

4 lanes × 25Gb/s = 100Gb/s line-rate class

These are lane-rate descriptions, not a promise of application throughput. Ethernet encoding, FEC, protocol overhead, and the capabilities of the connected devices affect usable data transfer.

A 40GbE link and a 100GbE link therefore require compatible electrical signaling at both ends. A cable cannot convert four 10Gb/s lanes into four 25Gb/s lanes.

Cable types: DAC, AOC, and fiber

Passive DAC

A passive direct-attach copper cable has QSFP-family ends connected by a copper assembly. It does not contain active signal-conditioning electronics in the cable.

Choose a passive DAC when:

• The equipment is close enough for the cable's specified reach
• Both ports support the cable's generation and coding
• Low cost and low power are priorities
• The cable can be routed without exceeding its bend or weight limits

Passive DACs are usually the simplest option for short rack or adjacent-rack connections. Their supported length is product-specific, so do not select one based only on “QSFP+” or “QSFP28” on the label.

A longer passive cable is not automatically equivalent to an active one. Copper loss, host equalization, cable construction, and port support all matter.

Active DAC

An active DAC includes electronics that condition or compensate for the signal. It can support cable assemblies that are not suitable as passive copper, but it consumes power and may have stricter compatibility requirements.

Check:

• Whether the switch or adapter accepts active DACs
• The cable's documented generation and lane rate
• Power and thermal limits at the port
• Vendor coding and firmware behavior
• The specified maximum length for that exact part

Active DAC should not be treated as a universal signal converter. It still needs compatible electrical signaling at both ends.

AOC

An active optical cable integrates optical conversion and the fiber between two fixed ends. It is useful when a single preterminated assembly is easier to deploy than separate transceivers and a fiber patch connection.

An AOC can be a practical choice for:

• Longer in-row or cross-row links than the chosen copper assembly supports
• Lower cable weight and easier routing
• Fixed QSFP+ to QSFP+ or QSFP28 to QSFP28 connections
• Installations where separate optics are unnecessary

The ends are permanently associated with the cable. If one end fails or the link needs a different connector arrangement, you generally replace the assembly rather than swapping an individual transceiver or patch lead.

Transceiver-based fiber

With a fiber link, each device uses a compatible QSFP+ or QSFP28 optical transceiver, and the transceivers connect through an appropriate fiber assembly.

This approach costs more components but provides more flexibility:

• Optics can be replaced independently
• Fiber patch lengths and routing can be changed
• The same fiber infrastructure may support different optical modules later
• Multimode and single-mode choices can be made according to the link design

The fiber connector and fiber type must match the selected optical modules. For example, an optic designed for a multi-lane MPO-style connection should not be paired with an arbitrary duplex patch cable. Always use the transceiver datasheet and the fiber plant's polarity and connector requirements.

Breakout cables and lane mapping

Breakout is where many apparently compatible QSFP cables fail.

Common 40GbE breakout

A typical QSFP+ breakout assembly maps one 40GbE port to four 10GbE interfaces:

1 × QSFP+ 40GbE = 4 × SFP+ 10GbE

The switch must support 40GbE breakout, and the four remote ports must support the matching 10GbE mode. The cable's lane order and end labeling must match the equipment's breakout requirements.

Common 100GbE breakout

A typical QSFP28 breakout assembly maps one 100GbE port to four 25GbE interfaces:

1 × QSFP28 100GbE = 4 × SFP28 25GbE

This requires a QSFP28 port and four compatible 25GbE endpoints, plus switch software and hardware support for the breakout configuration.

Do not assume cross-generation breakout

A QSFP28-to-four-SFP+ cable is not automatically a 40GbE breakout cable, and a QSFP+ breakout cable is not a 100GbE-to-4×25GbE assembly. Some platforms support unusual or mixed modes, but those modes must be explicitly documented.

Confirm all of the following:

• Parent port mode
• Remote port type
• Per-lane speed
• Lane mapping and polarity
• FEC requirements
• Whether the cable is passive, active, or optical
• The switch's breakout command or port-group configuration

A cable can be electrically wired correctly and still fail if the switch is left in its default single-port mode.

Vendor coding and interoperability

Many switches, network adapters, and cables use module identification data stored in an EEPROM. The host may inspect this information before enabling the port.

Compatibility problems can occur when:

• The cable is coded for a different vendor
• A switch rejects third-party module identification
• The host expects an optical module but receives a DAC or AOC
• The cable reports a different lane rate or media type than the port expects
• A firmware update changes module validation behavior
• Both ends accept the connector but not the same operating mode

When buying a third-party cable, look for explicit compatibility with the exact switch, NIC, or storage adapter family. “QSFP28 compatible” is less useful than a documented part number or compatibility list for the host device.

Also check whether the equipment requires vendor-coded ends individually. A custom-coded assembly may be needed even when the underlying electrical standard appears correct.

Common compatibility traps

Plugging a QSFP28 cable into a QSFP+ link and expecting 100GbE

The cable does not upgrade the port. If the host ports are 40GbE-only, a QSFP28 label cannot create 100GbE signaling.

Treating QSFP+ and QSFP28 as different connector shapes

They are closely related connector families. Mechanical fit is only the first check; protocol, lane speed, coding, and port configuration remain decisive.

Buying the wrong breakout direction

A cable described as “QSFP28 to 4 × SFP28” has one 100GbE parent side and four 25GbE child sides. It is not the same as a QSFP28-to-QSFP28 cable, nor is it interchangeable with a QSFP+ to 4 × SFP+ assembly.

Assuming a DAC can reach as far as an AOC

DAC, active DAC, and AOC have different electrical, optical, power, and distance characteristics. Compare the specified reach of the exact part, not just the connector family.

Mixing a cable and separate optics incorrectly

A transceiver-based fiber link needs compatible optics at both ends and matching fiber. An AOC already contains its optical ends; it is not a substitute for any arbitrary QSFP optic and patch cable combination.

Ignoring FEC and port configuration

Especially on higher-speed links, both ends may need compatible FEC and lane settings. A physically correct connection can remain down until the port group, speed, breakout, or FEC configuration is changed.

A practical buying checklist

Before ordering a QSFP+ or QSFP28 cable, record:

• Host port: exact switch, NIC, storage adapter, or router model
• Target protocol: 40GbE, 100GbE, or a documented alternate mode
• Port configuration: single link or breakout
• Lane mapping: 4 × 10Gb/s, 4 × 25Gb/s, or another documented arrangement
• Cable type: passive DAC, active DAC, AOC, or fiber with separate transceivers
• Required distance: include routing slack and patch-panel needs
• Remote connector: QSFP+, QSFP28, SFP+, SFP28, or optical connector
• Fiber requirements: multimode or single-mode, connector type, polarity, and optic compatibility
• Vendor coding: supported host brand, model family, and firmware expectations
• Power and thermal limits: particularly for active assemblies and optical modules
• Software support: breakout, FEC, speed selection, and port-group configuration
• Return policy: useful when host coding or undocumented interoperability is uncertain

A quick selection example

Suppose a switch has a documented 40GbE QSFP+ port and the remote device has four 10GbE SFP+ ports in the same rack. The appropriate starting point is a compatible QSFP+ to 4 × SFP+ 40GbE breakout DAC, provided the switch supports that breakout mode and the assembly's length is sufficient.

If the switch instead has a documented 100GbE QSFP28 port and the remote equipment exposes four 25GbE SFP28 ports, look for a compatible QSFP28 to 4 × SFP28 100GbE breakout assembly.

For a longer link, compare an AOC with a pair of supported optical transceivers and the required fiber. Do not choose solely by the fact that all options use a QSFP-family connector.

When you are ready to compare compatible assemblies, browse networking cables. You can also browse all cables if the installation may require fiber patch leads, adapters, or related connectivity hardware.