The decision that actually matters is not whether to run QSFP28. It is which variant to run. SR4, LR4, and CWDM4 each solve a different distance and fiber problem. Get the wrong one and you are either overpaying for reach you do not need or running into link budget failures on runs that are longer than your module was rated for.
This guide breaks down each variant by reach, fiber type, power budget, and deployment fit so you can specify the right module before you order.
Reach: Up to 100M on OM4 multimode fiber (70M on OM3)
SR4 uses four parallel 25G lanes over MPO-12 multimode fiber. It is the default choice for top-of-rack to aggregation switch connections, server-to-leaf links, and any run that stays within a single building or row.
Key specs at a glance:
SR4 is the lowest-cost 100G QSFP28 option and the most commonly stocked. If your runs are under 100M and you already have OM3 or OM4 plant in the building, SR4 is the correct answer. There is no reason to pay for single-mode capability you will never use.
One thing to watch: SR4 requires MPO trunk cables, not LC duplex. If your existing fiber plant is LC-based single-mode, SR4 is not a drop-in option. That is where CWDM4 or LR4 enters the picture.
Reach: Up to 10KM on G.652 single-mode fiber
LR4 uses four CWDM wavelengths (1271nm, 1291nm, 1311nm, 1331nm) multiplexed over a single LC duplex SMF pair. It is the right choice for data center interconnect (DCI) runs, campus backbone links, and any deployment where you need to cross a building boundary or a metropolitan campus.
Key specs at a glance:
LR4 costs more than SR4 and draws more power, but it solves the problem SR4 cannot: getting 100G across distances that multimode fiber cannot cover. If your data center interconnects a DR or a remote building over existing single-mode plant, LR4 is the standard choice.
Note that LR4 uses an integrated MUX/DEMUX inside the module itself. That means you get a single LC duplex interface rather than an MPO breakout, which simplifies patching considerably on long runs.
Reach: Up to 2KM on OS2 single-mode fiber
CWDM4 also uses four CWDM wavelengths over LC duplex SMF, but with a shorter reach specification and a lower power budget than LR4. It sits between SR4 and LR4 in both cost and reach, making it a practical option for intra-campus runs or data center builds where single-mode fiber is already installed but 10KM reach is unnecessary.
Key specs at a glance:
CWDM4 is common in hyperscale data center builds where the operator has standardized on single-mode fiber throughout the facility for future scalability, but where runs between racks and pods stay well under 2KM. It is also a cost-effective option when you need LC duplex SMF compatibility but cannot justify the price premium of LR4 for short-to-medium distances.
| Spec | SR4 | CWDM4 | LR4 |
|---|---|---|---|
| Max Reach | 100M (OM4) | 2KM | 10KM |
| Fiber Type | Multimode (OM3/OM4) | Single-mode OS2 | Single-mode OS2 |
| Connector | MPO-12 | LC Duplex | LC Duplex |
| Wavelength | 850nm x4 | CWDM 1271–1331nm | CWDM 1271–1331nm |
| Typical Power | ~1.5W | ~2.5–3.0W | ~3.5W |
| Relative Cost | Lowest | Mid | Higher |
| Best For | Intra-rack, TOR links | Campus SMF plant, pod-to-pod | DCI, campus backbone, 10KM runs |
Start with your fiber plant, not the module spec sheet.
If you have multimode fiber (OM3 or OM4) and runs under 100M: SR4 is the correct choice. It is the most cost-effective option and purpose-built for this environment.
If you have single-mode fiber already installed and runs under 2KM: CWDM4 gives you LC duplex SMF compatibility at a lower price point than LR4. Common in hyperscale builds and campuses that standardized on SMF for density and future scalability.
If you need to span a campus, connect two data center facilities, or run over existing metro SMF plant: LR4 at 10KM is the specification you need. The power premium over SR4 is real, but it is the only variant that covers the distance.
A few additional considerations worth factoring in:
Compatibility uncertainty is the most common reason engineers hesitate on third-party QSFP28 modules. The concern is valid, but it is solvable before you commit to an order.
Third-party compatible QSFP28 modules are programmed to match the vendor-specific EEPROM data that Cisco, Juniper, Huawei, Arista, and other switch vendors check during module initialization. A properly programmed module passes these checks and operates identically to the OEM part at the optical layer.
The practical steps to validate before purchasing:
At hytoptodevice.com, compatibility test videos and product downloads are available for pre-purchase validation across the QSFP28 catalog. If you are deploying on Cisco Nexus, Juniper QFX, Huawei CloudEngine, or Arista 7000-series hardware, verifying against published test data before ordering eliminates the compatibility guesswork.
OEM modules from Cisco run $200 to $500 or more per unit. Third-party compatible QSFP28 modules deliver the same IEEE-compliant optical performance at a fraction of that cost. On a 48-port 100G spine switch, that cost differential is significant at scale.
Q1: Can I mix SR4, LR4, and CWDM4 modules on the same switch?
A: Yes. QSFP28 is a form factor standard, not an optical standard. You can populate different ports with SR4, LR4, or CWDM4 modules depending on what each link requires. The switch treats each port independently.
Q2: Will a third-party QSFP28 module void my switch warranty?
A: In most jurisdictions, using third-party optics does not void hardware warranties under applicable law (the Magnuson-Moss Warranty Act in the US, for example). Switch vendors may attempt to restrict support for issues they attribute to third-party modules, but the hardware warranty on the switch itself is not voided by the optic choice.
Q3: What fiber do I need for QSFP28 SR4?
A: SR4 requires OM3 or OM4 multimode fiber with an MPO-12 connector. OM3 supports up to 70M; OM4 extends that to 100M. SR4 does not work on single-mode fiber.
Q4: Is CWDM4 compatible with LR4 on the same link?
A: No. CWDM4 and LR4 use the same four CWDM wavelengths but have different power budgets and reach specifications. You need matching variants at both ends of a link. Mixing them will likely result in a link that does not come up or operates outside its power budget.
Q5: What is the difference between QSFP28 and QSFP+ at 100G?
A: QSFP+ is a 40G form factor. QSFP28 is the 100G variant of the same physical housing. The two are not electrically compatible at the transceiver level, though some switches have ports that accept both form factors with appropriate module selection.
Q6: Do QSFP28 modules support digital diagnostics monitoring (DDM)?
A: Yes. All standard QSFP28 modules support DDM (also called DOM), which gives you real-time TX/RX power, temperature, voltage, and bias current readings via the switch management interface.
Q7: How do I know if a third-party QSFP28 module is compatible with my Cisco Nexus switch?
A: Check the module's datasheet for the specific Nexus platform and NX-OS version it has been tested against. Review any available compatibility test videos showing the module initializing on that hardware. Hytoptodevice publishes this validation material at hytoptodevice.com to support pre-purchase due diligence.
SR4, LR4, and CWDM4 each serve a distinct role in a 100G data center build. Match the variant to your fiber plant and your reach requirement, not the other way around. SR4 for multimode intra-facility runs, CWDM4 where single-mode is installed and distance stays under 2KM, LR4 when you need to cover 10KM over SMF.
If you are speccing a 100G upgrade or expanding an existing spine-leaf fabric, Hytoptodevice carries the full QSFP28 catalog with compatibility test videos and product downloads to support your validation process. Sign up at hytoptodevice.com to access the catalog and get the specs your procurement team needs.
