SFP and SFP+ Transceiver Modules

Fiber type and wavelength must agree at both ends

A link works only when both modules use the same type and wavelength. Multi-mode fiber with 850 nm modules (SX at 1G, SR at 10G) covers short runs inside a building, while single-mode fiber with 1310 nm LX or LR modules reaches 10 km, and 1550 nm types go further. You cannot mix SR with LR. Single-strand BiDi modules come as a matched pair, for example one transmitting 1270 nm and the other 1330 nm, so you need one of each. Rated distance also depends on the fiber grade.

Which cage, which speed, and will the switch accept it?

The cage decides the form factor: SFP for 1 Gbit, SFP+ for 10 Gbit, SFP28 for 25 Gbit and QSFP+ for 40 Gbit. An SFP module often fits an SFP+ port and runs at 1G if the switch allows it, but not the other way round. Some switches only accept modules coded for their own brand, so choose modules listed as compatible. For a short in-rack link, a passive twinax direct-attach cable avoids fiber altogether, and copper RJ-45 modules turn a port into twisted pair.

Questions about SFP and SFP+ Transceiver Modules

Can I use an SFP module in an SFP+ port?

Often yes, at 1 Gbit speed, provided the switch supports it and the port is configured for it. An SFP+ module in a plain SFP port will not work, because the port cannot run at 10 Gbit.

Single-mode or multi-mode fiber?

Use whichever your installed fiber is, because modules must match the cable. Multi-mode suits short distances such as inside a building or between racks, while single-mode covers kilometers. Mixing types gives a poor link or none. Check the cable's color and labeling if you are unsure.

Why does my switch reject a third-party module?

Some switch makers read an identification code in each module and reject unfamiliar ones. Choose a module coded for your switch brand, or check whether your switch has a setting to allow unsupported transceivers.