40G QSFP+ Transceivers Explained: LR4 vs SR4 vs SR-BD vs LX4

• LR4 runs single-mode fiber up to 10 km; SR4 runs multimode up to 150 m via MPO; SR-BD covers that same multimode reach over duplex LC; LX4 works either fiber type up to 2 km.
• Only LR4 and SR4 are official IEEE 802.3ba PMDs. SR-BD and LX4 are vendor/MSA designs built on the same form factor, not ratified IEEE standards themselves.
• Both ends of a link need matching transceiver types. A 40G QSFP+ optic usually works in a 100G QSFP28 port at 40G, but not the reverse.
• Match the transceiver to the fiber already in the ground, not the other way around, that’s the fastest way to narrow down which one you need.

Every 40G QSFP+ transceiver does the same job, moving data at 40 Gbps line speed over fiber, but the fiber type, connector, and reach each one needs are different enough that picking the wrong one means a link that won’t come up. QSFP-40G-LR4, QSFP-40G-SR4, SR-BD, and LX4 cover four different combinations of single-mode versus multimode fiber, reach, and cabling. Here’s what separates them and how to tell which one your link actually needs.

What is a 40G QSFP+ transceiver?

A 40G QSFP+ transceiver, often just called a QSFP module or QSFP+ module, is a pluggable optical module that fits a switch or router’s QSFP+ port and converts 40 Gigabit Ethernet electrical signals into optical signals for transmission over fiber. All 40G QSFP+ optics share the same form factor and electrical interface, so the port itself doesn’t determine reach or fiber type, the specific module you plug in does.

That’s why “40G QSFP+” isn’t really one product, it’s a form factor that a handful of different optical standards fit into. LR4, SR4, SR-BD, and LX4 are the four you’ll run into most often, and each one trades off reach, fiber type, and cabling cost differently.

What standard do 40G QSFP+ transceivers follow?

Every 40G QSFP+ module, including Cisco’s own QSFP-40G-LR4, follows the same QSFP+ MSA (SFF-8436), the multi-source agreement that defines the physical, electrical, and digital diagnostic monitoring (DOM) interface every vendor builds to, which is why the same module works across switch models from different vendors. The optical side of LR4 and SR4 follows IEEE 802.3ba, which specifies things like maximum bit error rate (BER) and required power margin over the rated distance.

IEEE 802.3ba formally defines four 40G PMDs: 40GBASE-KR4 (backplane), 40GBASE-CR4 (copper cable), 40GBASE-SR4 (multimode fiber), and 40GBASE-LR4 (single-mode fiber).

SR-BD and LX4 aren’t among those four officially defined PMDs. Both are vendor and MSA-designed optics built on the same QSFP+ form factor and generally engineered to meet 802.3ba’s electrical and BER framework, but they were never ratified as IEEE standards in their own right, they exist because SR4’s reach and LR4’s fiber requirements didn’t cover every real-world cabling scenario. Worth knowing if you’re specifying by standards compliance rather than just part number.

A Cisco QSFP-40G-LR4 module, a Finisar-made LR4, and a third-party LR4 are all mechanically and electrically interchangeable for exactly this reason, MSA compliance is what makes cross-vendor compatibility possible in the first place. What differs is the vendor coding layer that switch software checks separately from the physical standard, which is why a switch sometimes flags a fully IEEE 802.3ba-compliant device as “unsupported” even though it works electrically.

IEEE 802.3ba specifies these links to run at a maximum BER of 10⁻¹², the performance threshold that keeps a 40G link error-free in practice.

Every 40G QSFP+ module also includes DOM, giving you real-time monitoring of transmit and receive power, temperature, and bias current, useful for catching quality or reliability issues, and confirming you’ve got enough power margin, before they cause an outage. In practice, compatibility for QSFP-40G-LR4 procurement comes down to two separate things: MSA compliance for the physical and optical layer, and vendor coding for switch software compatibility.

What’s the difference between QSFP-40G-LR4 and QSFP-40G-SR4?

QSFP-40G-LR4 runs over single-mode fiber (SMF) up to 10 km using a duplex LC connector and CWDM technology that multiplexes four wavelengths onto one fiber pair. QSFP-40G-SR4 runs over multimode fiber (OM3/OM4) up to roughly 100-150 meters using an MTP/MPO-12 connector and parallel optics, with each of the four lanes on its own fiber strand instead of being multiplexed.

LR4 is the long-reach option built for single-mode links between buildings, floors, or data center rows. SR4 is the short-reach option built for dense, cost-sensitive intra-data-center links where multimode fiber and parallel optics keep the per-link cost down. If your run is under 150 meters and you’re already on multimode, SR4 is usually the cheaper choice; past that distance, LR4’s single-mode reach is the only one of the two that works.

What is 40GBASE-LR4 used for?

40GBASE-LR4 (QSFP-40G-LR4) is used for long-reach single-mode links up to 10 km, the kind of distance that shows up in enterprise core links, campus backbones, data center interconnects between buildings, and metro or carrier-edge links. Its duplex LC connector means it uses the same simple two-fiber cabling as most single-mode infrastructure already in place.

Because it only needs a single fiber pair instead of a 12-fiber MPO trunk, LR4 is also the easier optic to deploy when you’re running new single-mode fiber and want to keep cabling simple rather than provisioning parallel strands you don’t need at 10 km distances.

Browse compatible LR4 transceivers, backed by our 1-year hardware warranty and compatible across major switch platforms.

What is 40GBASE-SR4 used for?

40GBASE-SR4 (QSFP-40G-SR4) is used for short multimode links, typically top-of-rack to spine or switch-to-switch links that connect racks within the same data center row, where the distance stays under about 100-150 meters. It uses an MTP/MPO-12 connector and parallel optics instead of wavelength multiplexing.

SR4’s tradeoff is cabling. Because it runs parallel lanes over separate strands, it needs a 12-fiber MPO trunk rather than a simple duplex pair, which means more fiber management in the rack but a lower cost per transceiver than single-mode optics. Only 8 of those 12 fibers actually carry signal, 4 for transmit and 4 for receive, the remaining 4 positions are unused on a standard 40G SR4 link.

Browse compatible SR4 transceivers, ready to ship and covered under our standard hardware warranty.

What is 40GBASE-SR-BD and when should I use it instead of SR4?

40GBASE-SR-BD (BiDi) is a multimode 40G optic that transmits and receives on two different wavelengths over a single duplex LC fiber pair, instead of the eight fibers an MPO-based SR4 link needs. Use it when you’re upgrading existing duplex multimode cabling, like a former 10G link, to 40G without pulling new MPO trunks.

This is the one to reach for when your building already has duplex multimode fiber in the walls and re-cabling to 12-fiber MPO isn’t practical. The reach is comparable to SR4: about 100 m on OM3 and up to 150 m on OM4, the same fiber-grade dependency SR4 has. The cabling requirement is a fraction of SR4’s, though.

Contact us about 40G SR-BD transceivers, warrantied from the date of delivery, if you’re upgrading existing duplex multimode runs.

What is 40GBASE-LX4 and how is it different from LR4?

40GBASE-LX4 (QSFP-40G-LX4) is a dual-media optic that works over either multimode fiber (roughly 100-150 meters) or single-mode fiber (SMF, up to about 2 km), using the same duplex LC connector and CWDM approach as LR4 but tuned for shorter reach. It’s built for mixed-fiber deployments or transitional infrastructure where a link might run over either fiber type.

LX4 sits between SR4 and LR4: shorter reach than LR4’s 10 km, but with LR4’s single-mode capability that SR4 doesn’t have, plus multimode support SR4-only links can’t offer. It’s a useful pick when you’re not fully certain which fiber type a given run will end up using, or when a campus is mid-migration from multimode to single-mode. Like SR4 and SR-BD, LX4’s multimode reach depends on fiber grade, and vendor datasheets vary more here than they do for SR4, so confirm the OM3 vs. OM4 figure for the specific module before ordering rather than assuming the general 100-150 m range applies to your exact fiber.

Browse compatible LX4 transceivers for flexible multimode or single-mode runs under 2 km, all under warranty.

What are the exact specs for each 40G QSFP+ transceiver type?

QSFP-40G-LR4 runs single-mode fiber up to 10 km on a duplex LC connector. QSFP-40G-SR4 runs multimode fiber up to 100-150 m on an MTP/MPO-12 connector. 40GBASE-SR-BD runs multimode fiber up to 100-150 m on a duplex LC connector. QSFP-40G-LX4 runs multimode or single-mode fiber up to 150 m or 2 km on a duplex LC connector.

Which 40G QSFP+ transceiver do I need?

Pick QSFP-40G-SR4 for short multimode links under 150 meters where you can run MPO cabling. Pick SR-BD if you need that same short multimode reach but only have duplex fiber available. Pick QSFP-40G-LR4 for single-mode links up to 10 km. Pick QSFP-40G-LX4 if the link might run over either fiber type or sits in the 150 m to 2 km range on single-mode.

The fastest way to narrow it down: check what fiber is already in the ground first, then match the transceiver to that, not the other way around. Re-cabling to fit a transceiver is almost always more expensive than picking the optic that fits the cabling you already have.

Still not sure which one fits? Contact us with your fiber type and distance and we’ll help you match the right module before you order.

40G QSFP+ Transceivers: Frequently Asked Questions

Are QSFP-40G-LR4 transceivers compatible across Cisco, Arista, and Juniper switches?

Third-party QSFP-40G-LR4 modules are typically coded to work across multiple switch platforms, including Cisco switches, Arista, and Juniper, as long as the transceiver is properly programmed for that vendor. Native OEM transceivers are locked to their own platform by default. In Cisco switches specifically, a properly coded third-party module will pass the same optics validation as an OEM one, as long as it’s programmed with a vendor ID that the device recognizes.

Can I use a 40G QSFP+ transceiver in a 100G QSFP28 port?

Usually yes, in one direction. QSFP+ and QSFP28 share the same mechanical form factor, and most modern switches with QSFP28 ports are backward-compatible with 40G QSFP+ optics, the port simply negotiates down and runs the link at 40G. This is a common way to keep using existing 40G optics after upgrading to QSFP28-capable switches.

It doesn’t work the other way: a 100G QSFP28 transceiver won’t run in a legacy 40G QSFP+ port, since that port’s electronics can’t drive the faster per-lane signaling QSFP28 needs. Always check your specific switch’s datasheet before assuming backward compatibility, since it depends on the ASIC and firmware, not just the port shape.

Do both ends of a link need the same transceiver type?

Yes, in almost all cases. Both ends of a 40G link need matching transceiver types (LR4 to LR4, SR4 to SR4, and so on) since the wavelength, fiber type, and connector all have to align for the link to establish.

Is 40G QSFP+ being phased out in favor of 100G?

40G is still widely deployed and supported, but most new data center builds are moving to 100G QSFP28 or higher for new links. 40G QSFP+ remains a practical, lower-cost option for existing infrastructure and links that don’t need the extra bandwidth.

What warranty comes with these transceivers?

Network Republic covers end-user hardware, whether New, New Open Box, New Pull, Refurbished, or Refurbished by Manufacturer, with a 1-year warranty from the date of delivery for defects under normal use. Reseller and wholesale orders carry a 30-day warranty limited to functional issues and dead-on-arrival units, so inspect and test within that window.

Bottom line

Match the transceiver to the fiber you already have, not the other way around, and the LR4 vs SR4 vs SR-BD vs LX4 choice mostly makes itself: multimode and short means SR4 or SR-BD depending on your cabling, single-mode and long means LR4, and anything uncertain or mixed leans toward LX4.

Request a quote on the full 40G QSFP+ transceiver lineup, backed by a 1-year hardware warranty across all four types.