By Dustin Guttadauro, Product Line Manager - Telecom & Fiber, Infinite Electronics
Although 100G and higher-speed Ethernet are increasingly common in modern data centers, 40G QSFP+ connectivity remains relevant in many existing aggregation and switching environments. Organizations with established 40G infrastructure may not need to replace it immediately. Instead, the key question is whether the current 40G architecture continues to meet bandwidth requirements and whether it aligns with the organization's next network refresh.
40G QSFP+ provides a compact interface for 40G Ethernet and supports several connectivity options, including 40GBASE-SR4 over multimode fiber, 40GBASE-LR4 over single-mode fiber, direct attach copper (DAC), and active optical cables (AOCs). It can also support a 4×10G breakout configuration in compatible systems, allowing one 40G port to connect to four 10G interfaces.
For teams maintaining a 40G network, the decision typically comes down to three choices: continue using 40G, use breakout to extend the usefulness of existing ports, or begin migrating selected links to 100G.
Key Takeaways
- 40G QSFP+ remains useful in many existing data center networks.
- 40GBASE-SR4 is commonly used for short-reach multimode fiber connections.
- 40GBASE-LR4 is designed for longer-reach single-mode fiber applications.
- 40G DACs are suited to short copper connections.
- 40G AOCs provide integrated optical connectivity for short-to-moderate links.
- Compatible QSFP+ systems can support 4×10G breakout.
- QSFP28 provides a path from 40G toward 100G connectivity.
- A 40G-to-100G migration should evaluate switches, optics, fiber, connectors, and cabling together.
- Not every existing 40G link needs to be upgraded immediately.
- A phased migration can allow organizations to prioritize high-utilization links and equipment nearing the end of its lifecycle.
What Is 40G QSFP+?
QSFP+, or Quad Small Form-factor Pluggable Plus, is a compact transceiver form factor commonly used for 40G Ethernet connections. A typical 40G QSFP+ implementation uses four optical or electrical lanes operating at approximately 10G per lane. Combining those lanes provides a 40G-class connection while allowing the transceiver to maintain a relatively small physical footprint. The four-lane architecture also enables breakout applications. In a compatible configuration, one 40G QSFP+ port can connect to four 10G SFP+ ports:
40G QSFP+ → Breakout Cable → 4 × 10G
This makes QSFP+ useful in networks where aggregation or uplink capacity has moved to 40G while some downstream equipment continues to operate at 10G.
Common 40G QSFP+ Connectivity Options
The appropriate QSFP+ implementation depends on distance, fiber type, connector configuration, and rack topology.
40GBASE-SR4
40GBASE-SR4 is designed for short-reach multimode fiber applications and commonly uses parallel optical transmission with an MPO/MTP-style connector. SR4 is particularly suited to data center environments where network devices are located within relatively short distances of one another.
Common applications include:
- Leaf-to-spine connections
- Rack-to-rack links
- Switch-to-switch connections
- Short aggregation links
- High-density multimode fiber deployments
When deploying SR4, verify the installed multimode fiber, connector configuration, and required link distance before selecting the transceiver.
40GBASE-LR4
40GBASE-LR4 is intended for longer-reach connections and typically operates over single-mode fiber. Rather than using the parallel multimode architecture associated with SR4, LR4 uses multiple optical wavelengths over a duplex single-mode fiber connection. This makes LR4 useful when the link extends beyond the practical range of a short-reach multimode deployment or when the data center already uses single-mode fiber.
Typical applications include:
- Longer switch-to-switch links
- Data center backbone connections
- Campus connectivity
- Longer aggregation links
- Environments standardized on single-mode fiber
The actual reach depends on the specific transceiver and link budget, so the manufacturer's specifications should always be checked.
40G DAC vs. 40G AOC
Fiber transceivers are not the only way to create a 40G connection. For short links, DACs and AOCs can simplify installation.
40G Direct Attach Copper
A 40G DAC combines copper cable with integrated QSFP+ connectors. DACs are generally intended for short connections and can be useful for same-rack or nearby equipment.
Advantages include:
- Simple installation
- Integrated connectors
- No separate optical transceivers
- No separate fiber patch cable
- Straightforward rack-level deployment
- Useful for short switch-to-switch links
The main limitation is distance. DACs are designed for short-reach applications and may be less flexible than fiber when cable routing distances increase.
40G Active Optical Cable
A 40G AOC combines optical fiber with integrated active electronics at the cable ends. An AOC can provide greater reach than a copper DAC while retaining the simplicity of an integrated cable.
AOCs may be appropriate when:
- The link is relatively short
- A fixed cable length is acceptable
- Separate transceivers are not required
- Simplified installation is preferred
- Lower cable weight is useful for the application
Because the cable and optical ends are integrated, an AOC provides less component-level flexibility than separate transceivers and fiber assemblies.
4×10G Breakout: Extending the Value of 40G
One of the most useful capabilities of compatible 40G QSFP+ systems is 4×10G breakout. Instead of using one 40G port for a single connection, a breakout assembly can divide the connection into four 10G links:
40G QSFP+ → Breakout Assembly → 4 × 10G SFP+
This can be useful in aggregation environments where the upstream switch provides 40G capacity, but downstream equipment still has 10G interfaces. However, engineers should not assume that every 40G QSFP+ port supports breakout.
Before deployment, verify:
- The switch supports 4×10G breakout.
- The QSFP+ module supports the intended configuration.
- The breakout cable has the correct connector arrangement.
- The four 10G interfaces support the selected optics or cables.
- The switch software supports the required port mode.
- The physical media and link budget are appropriate.
This makes breakout a system-level compatibility decision rather than simply a cable-selection decision.
40G QSFP+ vs. QSFP28 100G
The transition from 40G QSFP+ to QSFP28 is an important consideration for organizations planning a network refresh. A typical 40G QSFP+ architecture uses four lanes operating around 10G each. QSFP28 is commonly used for 100G applications and uses four lanes operating around 25G each.
This provides a significant increase in aggregate bandwidth while maintaining a compact QSFP-family form factor. However, upgrading from 40G to 100G involves more than replacing the transceiver. Engineers should evaluate the switches, optics, fiber plant, connectors, breakout requirements, and connected equipment as a complete system.
Common 40G QSFP+ Mistakes
Choosing a module based only on speed
Two QSFP+ modules can both provide 40G connectivity while using different fiber, connector, and reach specifications.
Assuming SR4 and LR4 use the same infrastructure
SR4 commonly uses short-reach multimode fiber with parallel optics, while LR4 typically uses single-mode fiber and a duplex configuration.
Assuming every 40G port supports breakout
Breakout support depends on the specific switch and configuration.
Ignoring the existing cable plant during migration
Moving to 100G can require changes to connectors, fiber type, patching, or transceiver architecture.
Replacing all 40G links at once
A phased migration may be more practical when some 40G links remain well within capacity while others require immediate expansion.
40G QSFP+ remains a practical connectivity option for many established data center networks, particularly where existing hardware and fiber infrastructure continue to meet performance requirements. SR4 provides a common short-reach multimode option, LR4 supports longer single-mode links, while DACs and AOCs offer alternatives for shorter connections. The 4×10G breakout capability can also extend the usefulness of existing 40G aggregation equipment.
For organizations planning a major network refresh or experiencing growing uplink requirements, 100G and QSFP28 provide a path toward higher capacity. The decision should ultimately consider bandwidth demand, equipment lifecycle, physical-layer compatibility, breakout requirements, migration cost, and future network architecture. By evaluating these factors together, data center teams can determine which 40G links should remain in service, which can benefit from breakout, and which should become candidates for a move to 100G.
How L-com Supports Data Center Connectivity
L-com provides connectivity products for data center and network applications. For projects that involve 40G or 100G connectivity, L-com offers fiber transceivers and active optical cables that can support a range of switch platforms, form factors, and data center architectures.
Frequently Asked Questions (FAQs)
1. What is 40G QSFP+?
40G QSFP+ is a compact transceiver form factor commonly used for 40G Ethernet connectivity. A typical implementation uses four lanes operating at approximately 10G each. It supports several connectivity options, including 40GBASE-SR4 over multimode fiber, 40GBASE-LR4 over single-mode fiber, direct attach copper (DAC), and active optical cables (AOCs).
2. What is the difference between 40GBASE-SR4 and 40GBASE-LR4?
40GBASE-SR4 is a short-reach implementation commonly used with multimode fiber and parallel optical transmission through an MPO/MTP-style connection. 40GBASE-LR4 is a longer-reach implementation that typically uses single-mode fiber and multiple wavelengths over a duplex fiber connection. The actual reach depends on the specific transceiver and link budget.
3. Can 40G QSFP+ break out to four 10G ports?
Yes, in compatible systems. A 40G QSFP+ port can support a 4 × 10G breakout configuration using an appropriate breakout cable and a supported switch configuration. Not every 40G QSFP+ port supports breakout, so verify the switch, module, cable, and software before deployment.
4. Should I use a 40G DAC or a 40G AOC?
A DAC combines copper cable with integrated QSFP+ connectors and suits short connections, such as same-rack or nearby equipment. An AOC combines optical fiber with integrated active electronics and can provide greater reach than a DAC. Both are fixed-length, integrated options, so they offer less flexibility than separate transceivers and fiber assemblies.
5. What is the difference between QSFP+ and QSFP28?
QSFP+ is commonly used for 40G and uses four lanes operating at around 10G each. QSFP28 is commonly used for 100G and uses four lanes operating at around 25G each. Both are compact QSFP-family form factors, but upgrading from 40G to 100G also requires evaluating switches, optics, fiber, connectors, and cabling together.
6. Do I need to replace all my 40G links with 100G?
No. Not every existing 40G link needs to be upgraded immediately. A phased migration lets organizations prioritize high-utilization links and equipment nearing the end of its lifecycle, while other 40G links remain in service or use breakout to extend the usefulness of existing ports.