The Tool Desk
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What QSFP28 and CFP4 describe
Both names refer to the physical and electrical module-and-host interface family. They do not, by themselves, specify the optical wavelength, reach, fiber type or connector. The CFP4 MSA covers 40Gbit/s and 100Gbit/s applications, while QSFP28 is a compact form factor used for 100G modules.
The optical standard is a separate choice. For example, the CWDM4 MSA describes an interface built with four 25 Gbps optical lanes and allows initial implementations in either CFP4 or QSFP28. That is why the form factor alone cannot tell you whether an optic will suit a particular fiber link.
Side-by-side differences
| Comparison | CFP4 | QSFP28 | What to check |
|---|---|---|---|
| Meaning | CFP-family form factor; CFP4 MSA covers 40G and 100G applications. | QSFP-family form factor used for 100G modules. | Confirm the host has the matching cage and supports the exact module. |
| Optical interface and reach | Depends on the specific optic, such as LR4 or CWDM4. | Depends on the specific optic, such as LR4 or CWDM4. | Compare the same optical standard and required link distance, not just the form factor. |
| Physical size | TE Connectivity lists CFP4 width as 21.7 mm in its family comparison. | QSFP is a compact port family; a comparable width measurement is not stated in the cited source. | Actual port density depends on the chassis and host design. |
| Power | TE lists a CFP4 family maximum-power range of 1.5–9 W; this is not a measurement for every CFP4 optic. | Cisco’s portfolio table lists 3.5 W for the QSFP-100G-LR4-S example. | Check the exact module’s power and the host’s thermal and power limits. |
| Compatibility | Requires a CFP4-compatible host port and platform support. | Requires a QSFP28-compatible host port and platform support. | Check the vendor’s compatibility matrix, including software or firmware requirements. |
Do the modules fit the same ports?
No. Hot-pluggable means a module can be inserted or removed while the compatible host system is operating; it does not make different form factors physically interchangeable. The CFP MSA’s hot-pluggability statement applies to CFP4 modules and their host systems, not to QSFP28 ports. Consult the documentation for the exact switch, router, NIC or line card and the manufacturer’s supported-optics list.
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Compare matching optical standards, not just 100G labels
LR4-to-LR4 is a useful like-for-like example. Cisco specifies its CFP-100G-LR4 for up to 10 km over G.652 single-mode fiber, with dual SC/PC connectors. Cisco specifies its QSFP-100G-LR4-S for up to 10 km over a standard pair of G.652 single-mode fibers, with duplex LC connectors. The cited QSFP portfolio comparison lists 3.5 W for that model.
The CFP example is a full-size CFP module, not CFP4. Cisco’s CFP datasheet lists 24 W maximum power and dimensions of 144.8 × 82 × 13.6 mm for that CFP module. Those figures must not be treated as CFP4 specifications. The two LR4 examples illustrate that similar stated reach does not guarantee the same connector arrangement, power draw or host compatibility.
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If your host supports QSFP28 and the link calls for LR4 over single-mode fiber, a QSFP28 100GBASE-LR4 transceiver is a relevant module type to investigate. Verify the exact host model’s supported optics and module coding, and confirm both ends’ connector and fiber arrangements before buying.
Check FEC and link requirements
Forward error correction (FEC) behavior can vary by optical standard and platform. In Cisco’s cited portfolio examples, the host uses RS-FEC for SR4 and CWDM4, while the listed LR4 example uses no FEC. Treat this as model- and platform-specific guidance, not a universal rule for every module labeled SR4, CWDM4 or LR4. Check the exact optic and host documentation for FEC settings and link-budget requirements.
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Selection checklist
- Identify the host port. Record the exact switch, router, NIC or line-card model and whether the port accepts CFP4 or QSFP28.
- Verify platform support. Check the manufacturer’s supported-optics matrix and any stated software or firmware requirements.
- Choose the optical interface and reach. Select the needed standard and distance; do not infer either from “100G” or from the module form factor.
- Match the physical link. Confirm fiber type, connector, wavelength, polarity and any minimum-distance constraints at both ends. Allow for differences such as SC/PC versus LC when planning patching.
- Confirm signal requirements. Check link budget and the host’s required FEC behavior for the selected module.
- Check power and thermal limits. Use the exact module’s specifications and confirm the port, line card and chassis can support it.
- Check both endpoints. Verify that each host supports its installed optic and that the two optics and the patching arrangement can form the intended link.
Why power and density claims need context
Form factor alone does not guarantee a particular module power draw or chassis density. The cited figures use different scopes: TE Connectivity gives a 1.5–9 W maximum-power range for the CFP4 family, while Cisco lists 3.5 W for one QSFP LR4 product. Cisco’s 24 W maximum applies to the full-size CFP-100G-LR4, not CFP4. These examples are useful for checking specific products, but they are not a controlled comparison of equivalent modules or a promise about every design.
Similarly, a module’s width does not establish how many ports a particular switch can fit. Host-panel layout and chassis design determine practical port density; consult the platform specifications rather than extrapolating from the form-factor name.
Quick Recap
Best Value
- 5-in-1 Connectivity: Equipped with a 4K HDMI port, a 5 Gbps USB-C data port, two 5 Gbps USB-A ports, and a USB C 100W PD-IN port. Note: The USB C 100W PD-IN port supports only charging and does not support data transfer devices such as headphones or speakers.
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