Two QSFP28 modules can fit the same ports and still fail to interoperate because they use different lane structures, modulation, wavelengths, connectors or FEC requirements. A correct selection starts with the installed link, not a generic “100G” label.

Collect these facts before comparing module prices:

• Switch, router or network-adapter model at each endpoint
• Line card, port type and software or firmware release
• Native 100G or breakout operation
• Required reach based on the measured cable route
• Existing fiber: multimode or single-mode
• Connector and patch-panel format: duplex LC or parallel MPO
• FEC required or supported by each host and interface
• Environmental temperature class and power limits
• Whether the remote optic is fixed or can be changed
• Planned migration to 400G or another architecture

If any item is unknown, mark it for verification. Do not choose an optic only because its advertised reach exceeds the route length.

The following table describes common planning categories. It is not a substitute for the exact host support matrix or module datasheet.

“Around” reflects the common design class. Use the exact module datasheet, platform matrix and fiber route to confirm permitted reach and optical limits.

Both can be described as 100G over duplex single-mode fiber with LC connectors and a 10 km-class reach, but they use different optical architectures.

• LR4 carries the 100G signal across four optical wavelengths.
• LR1 carries the 100G signal on a single optical lane using a different signalling architecture.

Matching speed, connector and nominal reach does not make them interoperable. Use LR4 with a verified LR4 remote interface and LR1 with a verified LR1 remote interface unless authoritative documentation explicitly supports another combination.

If an LR4 module faces an LR1 module, cleaning the connectors or changing the coding profile will not make the optical interfaces compatible.

Short-reach selection depends heavily on the installed fiber and connector system.

SR4

SR4 commonly uses parallel multimode lanes through an MPO connector. Verify the OM grade, polarity method, connector gender, trunk construction and whether the cabling supports the intended application.

SR1

SR1 commonly uses duplex multimode fiber with LC connectors. It is not simply an SR4 module with another connector. Confirm that both hosts support the single-lane interface and required FEC.

PSM4

PSM4 commonly uses parallel single-mode fiber through MPO. Although the module can resemble SR4 mechanically, its fiber medium and optical application differ. Do not connect PSM4 to an MMF SR4 plant.

Choose the interface that matches the actual cable plant or include the cost and operational impact of changing the plant.

FEC is part of many newer 100G single-lane and short-reach interfaces. A link can remain down or accumulate errors when the endpoints use different FEC settings or when the host does not support the required mode.

For each endpoint, record:

1. Supported 100G PMD interface
2. Required or optional FEC mode
3. Native 100G or breakout configuration
4. Lane mapping for breakout
5. Supported software release
6. Transceiver coding profile and host acceptance

Do not disable FEC merely to force link-up unless the exact interface and platform documentation support that configuration.

A QSFP28 port may support native 100G, four 25G lanes or another mode, depending on the platform. Breakout requires more than a compatible cable:

• The switch port must support the intended breakout mode.
• The remote ports must support the resulting lane speeds.
• Lane order and mapping must match the cable or optical topology.
• FEC and speed settings must be correct on all child interfaces.
• The transceiver must support the intended channelized operation.

Check the exact platform documentation before ordering splitter cables or parallel optics. Form factor and lane count do not prove that a port can be channelized.

Module price is only one part of a 100G decision.

Do not publish a fixed savings percentage without quotations for the exact modules, cable plant and labour scope.

For a selected module pair, calculate the link budget from authoritative specifications:

• Minimum transmitter power
• Minimum receiver sensitivity
• Maximum receiver input or overload limit
• Fiber attenuation at the relevant wavelengths
• Connector, splice and passive-component losses
• Engineering margin appropriate to the project

Long-reach optics on a short low-loss route can require an overload check. Short-reach optics cannot be extended merely by using cleaner fiber. Use the transmitter and receiver limits for the exact part numbers.

A module can present acceptable identity data to the host but still use the wrong optical interface. Conversely, an optically suitable module can be rejected by the host because of coding or platform policy.

Verify three independent layers:

1. Host recognition and acceptance
2. Ethernet interface and FEC interoperability with the remote end
3. Fiber, connector, polarity and optical budget

Axonode compatibility language should remain narrow: coded for a recorded vendor or platform profile, or validated in an exact configuration under recorded test conditions. Do not describe a generic QSFP28 as universally compatible.

Before quantity deployment:

1. Record both endpoint models and software releases.
2. Confirm the selected PMD interface at both ends.
3. Record module part numbers and coding profiles.
4. Verify fiber type, connector, polarity and route loss.
5. Configure speed, breakout and FEC.
6. Confirm recognition and DOM fields where supported.
7. Establish the link and inspect error counters under representative traffic.
8. Save the validated BOM and test conditions.

A passing sample supports that recorded configuration. It does not prove operation on every host or future software release.

• Match the exact 100G interface at both ends.
• Confirm host, line card, port mode and software support.
• Match multimode or single-mode fiber and the correct connector.
• Verify MPO polarity, gender and lane mapping for parallel interfaces.
• Confirm FEC before ordering single-lane or breakout solutions.
• Calculate optical budget and receiver overload from exact specifications.
• Validate one representative link before ordering quantity.

Can a 100G LR4 module connect directly to a 100G LR1 module?

No, not by default. LR4 and LR1 use different optical architectures. Use matching, documented interfaces at both ends unless authoritative interoperability documentation confirms the exact combination.

Is every 100G QSFP28 compatible with every QSFP28 port?

No. The host must support the module type, coding profile, power class, PMD interface, port mode, FEC and software context. Mechanical fit is only one condition.

Should I choose SR4 or an LC-based short-reach optic?

Choose according to the installed cable plant, host support, reach, FEC, connector operations and upgrade plan. SR4 normally uses parallel MMF and MPO; LC-based short-reach interfaces can use a different lane architecture and are not automatic replacements.

Is CWDM4 the same as FR1 because both may cover a 2 km-class link?

No. They use different optical architectures and specifications. Match the exact interface at both ends and verify platform support rather than selecting by reach alone.

Can I use four 25G links instead of one native 100G link?

That is a topology and application decision. Four 25G links can provide aggregate capacity, but traffic distribution and single-flow behaviour depend on the network design. Confirm whether the application requires one native 100G Ethernet interface or a bundled set of 25G links.

Does receiving optical power prove the 100G interfaces match?

No. Optical energy can be present even when the endpoints use different PMDs, lane structures, modulation or FEC. Confirm interface identity, not only the power reading.

Axonode can help organize a 100G optics bill of materials and coordinate coding and validation requirements with vetted OEM manufacturing and testing partners. Share both endpoint platforms, software, port mode, required reach, installed fiber, connectors, measured route loss, FEC, breakout topology, quantities and future upgrade target.

Review the 100G and other optical transceiver options, see the Data Center and AI Cloud solution, browse the product catalog, or contact Axonode with the link details.