SFP vs SFP+ vs SFP28 vs QSFP28: Types and Compatibility

Choosing the wrong pluggable transceiver - or assuming two form factors are interchangeable - is a classic source of link-down mystery. SFP, SFP+, SFP28, QSFP+ and QSFP28 share the same MSA (multi-source agreement) ecosystem but differ in speed, lane count and, critically, compatibility rules. This guide compares the five form factors and answers the questions that actually come up on site: can an SFP28 plug into an SFP+ port, and what happens if it does?

Form Factor Reference Table

SFP was introduced in 2001 to replace GBIC at up to 1G; SFP+ (SFF-8431) brought 10G to the same physical shell; SFP28 kept the shell for 25G single-lane; QSFP families quadruple the lanes for 40G/100G in one slot, which is why 4x SFP+ optics can often be replaced by a single QSFP+ module.

Compatibility Rules That Matter

• 1G SFP in an SFP+ port: generally works - SFP+ ports are designed to accept SFP optics, and the link runs at 1G. Many 10G switches auto-negotiate down or need the port forced to 1G.
• SFP+ in an SFP port: do NOT do this - the reverse direction is not supported and can damage the module or port.
• SFP28 in an SFP+ port: the connectors and pinouts are mating-compatible, so a 25G SFP28 module inserted into an SFP+ (10G) port negotiates down and runs at 10G if the switch allows the speed. Conversely, a 10G SFP+ module in an SFP28 port works when the port is configured for 10G.
• QSFP28 vs QSFP+: same shell and management interface (SFF-8636); a QSFP28 port usually accepts QSFP+ modules at 40G, and QSFP28 100G modules do not drop into QSFP+ 40G ports.
• Breakout: QSFP28 ports commonly support breakout to 4x25G (or 4x10G) via DAC/fiber fan-out cables when the platform supports speed 4x25G port modes.

DAC, AOC and Optics

Not every pluggable is an optical module. DAC (direct attach copper) cables have transceivers molded into the ends - lowest cost and power for short racks (up to ~7m at 10G); AOC (active optical cable) extends the same plug-and-play model to tens of meters; standalone optical modules need fiber and are chosen by distance and fiber type (SR over multimode OM3/OM4, LR over single-mode OS2).

Practical Selection Advice

• Match the module's reach class to the link: don't pay LR prices for a 5 m rack link.
• Keep the same vendor/standard on both ends (SR-SR, LR-LR) and check DOM (digital optical monitoring) support if you plan link-budget monitoring.
• When a link fails after a swap, first suspect a form-factor or speed mismatch, then verify with the steps in our SFP transceiver troubleshooting checklist.

Choosing the optic is only half the job - the fiber plant behind it decides reach: see our CWDM vs DWDM comparison when wavelengths are involved, and the MTU mismatch troubleshooting guide for the frame-size issues that show up right after new optics go in.

原文链接：https://www.fs.com/eu-en/blog/sfp-sfp-sfp28-qsfp-qsfp28-what-are-the-differences-406.html