The cage takes the form factor the switch names — SFP, SFP+ and SFP28 among them — and the module must also match the rate and the medium at the far end. Some switches publish a compatibility list and refuse anything outside it.
A cage is a slot, and four separate things have to agree before a link comes up in it.
The form factor
The cage takes one family of module. SFP is the original small pluggable, SFP+ is the same outline at the higher rate, SFP28 higher again, and the QSFP family is a physically larger cage entirely. A switch names its cages, and a QSFP module does not go into an SFP cage at all.
Within the compatible outlines, the switch maker states which slower modules a faster cage accepts. Generally the faster cages take the slower modules; the specification sheet is where that is confirmed rather than assumed.
The rate
A module is built for a rate, and both ends of the link have to be running it. Two switches with SFP+ cages joined by modules of different rates do not negotiate a middle ground the way copper ports do — the pairing is fixed by the modules, and a mismatch simply does not come up.
The medium and the distance
This is the choice people make wrongly first.
Multi-mode fibre modules are the shorter-reach kind, and they are matched to a multi-mode cable of a stated grade. The reach is published against that grade, and it falls as the rate rises.
Single-mode fibre modules reach much further and are matched to single-mode cable. Some single-mode modules use a different wavelength in each direction so that one strand carries both, and those are sold and fitted as a pair.
Copper modules put an eight-position socket into the cage. They are convenient and they are among the most power-hungry things a cage can hold, which is why switch makers state which ones they support and how many may be fitted at once.
The two ends must match in medium, in grade of fibre and in wavelength. A module is not a translator.
The compatibility list
This is the part that is not a standard. Many switch makers program their equipment to check the identification data a module carries and to accept only modules on a published list. Some units allow the check to be relaxed; some do not.
There is nothing to work out here — the switch’s documentation states the policy, and it is the one line worth finding before buying modules from anywhere other than the switch maker.
Heat and power, which are part of the specification
Modules dissipate heat inside the cage, and a cage full of the hungriest kind is a real thermal load on a small switch. Makers publish a per-module allowance and sometimes a limit on how many high-consumption modules may be fitted. On a fanless unit this is not a footnote.
What to write down before ordering
Cage family as the switch names it, rate, medium and fibre grade at both ends, wavelength where single-mode is involved, connector type on the patch lead, and the switch maker’s compatibility policy. Six published fields, and between two units in one cabinet, the answer to all six may be a single direct-attach lead instead.
The questions that come up before an order
Will an SFP module work in an SFP+ cage?
Commonly yes, because the higher cages are generally specified to accept the slower modules, and the switch maker states it. The reverse does not hold: a module built for the faster rate in a cage that only supports the slower one is outside what either was specified for.
What is a direct-attach cable, and when is it the better answer?
A fixed lead with a module moulded on each end, sold in set lengths. Between two units in the same cabinet it is cheaper than two modules and a patch lead, draws less power and produces less heat. It cannot be re-terminated or extended, which is the trade.
Last reviewed 10 September 2026