SFP Modules Explained: SFP vs SFP+, Multimode vs Single-Mode, and How to Order the Right One
An SFP is the small module that turns a switch port into a fibre or long-copper link. Ordering the wrong one is the most common reason a new link stays dark, and the four things that decide it are speed, fibre type, distance and vendor coding. Here is how to get all four right the first time.
What an SFP actually is
A switch port is electrical. Fibre is light. An SFP is the small hot-swappable module that sits in the port and does the conversion, so that one switch can take copper, multimode fibre or single-mode fibre simply by changing the module.
That is also why the uplink ports on a good switch are SFP cages rather than fixed connectors: the switch does not need to know today whether the link to the next floor will be multimode, single-mode or a short copper run. You decide that with the module.
SFP, SFP+, SFP28, QSFP and GBIC
| Form factor | Speed | Typical use | Same size as SFP? |
|---|---|---|---|
| GBIC | 1 Gbps | Legacy equipment, largely retired | No, larger |
| SFP | 1 Gbps | Floor-to-floor links, small switches, older cores | Yes |
| SFP+ | 10 Gbps | Switch uplinks, servers, modern cores | Yes |
| SFP28 | 25 Gbps | Data centre server links | Yes |
| QSFP+ / QSFP28 | 40 / 100 Gbps | Data centre cores and spines | No, larger, four lanes |
Two rules worth memorising: an SFP+ port usually accepts a 1G SFP, but an SFP port never runs a 10G module; and a QSFP will not fit an SFP cage regardless of what the price looks like.
Multimode or single-mode?
| Multimode | Single-mode | |
|---|---|---|
| Core | Wider (50 or 62.5 micron) | Narrow (9 micron) |
| Typical patch cable colour | Aqua (OM3/OM4) or orange (OM1/OM2) | Yellow |
| Distance | Up to about 300 to 550 m at 10G | 10 km, 40 km, 80 km depending on optic |
| Module cost | Lower | Higher |
| Right for | Inside a building, floor to floor, rack to rack | Between buildings, campus links, operator handoff |
The module must match the fibre already in the wall. You cannot run a single-mode optic over multimode fibre or the reverse. If you do not know which is installed, look at the patch panel labelling and the colour of the existing patch leads, or ask the cabling contractor for the test results. Guessing here wastes a site visit.
Distances and the common part codes
| Code | Fibre | Nominal distance | Where it is used |
|---|---|---|---|
| SX | Multimode | Up to 550 m at 1G | Floor-to-floor links inside a building |
| SR | Multimode | 300 to 400 m at 10G | The standard 10G in-building uplink |
| LX / LR | Single-mode | 10 km | Between buildings, campus backbone |
| ER | Single-mode | 40 km | Long campus or metro links |
| ZR | Single-mode | 80 km | Operator and long-haul links |
| T / copper SFP | Cat6 RJ45 | 100 m | When an SFP cage must take copper |
| BiDi | Single strand | 10 to 40 km | Where only one fibre strand is available; ordered as a matched pair |
Examples from what we hold: a Cisco GLC-SX-MMD is a 1G multimode SX module; an HPE X132 10G SFP+ LC SR is the 10G multimode equivalent. Both need an identical module at the far end.
DAC and AOC cables: when you do not need transceivers
- DAC (Direct Attach Copper) is a fixed-length twinax cable with the transceiver ends built in, normally 0.5 to 7 m. For two switches in the same rack, or a switch to a server, it is cheaper than two optics plus a patch lead, draws less power and has nothing to get dusty.
- AOC (Active Optical Cable) is the same idea with fibre inside, for longer runs up to tens of metres, typically between racks.
- Limitations: fixed length, so measure before ordering; thicker and less flexible than a fibre patch lead, which matters in a tidy rack; and vendor coding applies at both ends, so a DAC between two different brands needs care.
- Rule of thumb: same rack, use DAC. Between floors, use fibre and a pair of transceivers.
Vendor coding and compatibility
Every module carries a small EEPROM chip identifying its vendor and part number. Switches read it, and three things can happen: the module is accepted, accepted with a warning, or refused.
- Original vendor modules always work and are the right answer where a support contract or tender demands it.
- Compatible modules coded for your switch brand are widely used, cost much less and work reliably. The coding must match the switch, not the fibre.
- Uncoded or wrongly coded modules are where projects go wrong. Some Cisco platforms will pass traffic on an unsupported module only after a service-unsupported-transceiver command, which is also a support conversation you may not want to have.
- Mixed ends are fine. A Cisco-coded module at one end and a Ubiquiti-coded module at the other will link happily, as long as speed, fibre type and wavelength match. The coding matters to the switch, not to the light.
When you order from us, tell us the exact switch model at each end. That single detail prevents nearly every compatibility problem.
How to order without getting it wrong
- Switch model at each end. Not the brand, the model: port capability and coding both depend on it.
- Speed of the link, 1G or 10G, and check that both ends support it.
- Fibre type installed: multimode or single-mode, confirmed from the patch panel or the cabling test report.
- Distance, the real cable route, not the straight line across the car park.
- Connector type, almost always LC duplex today; check whether the existing patch leads match.
- Quantity in pairs, plus at least one spare per link type on any site that matters.
Why the link is not coming up
| Symptom | Usual cause |
|---|---|
| No light at all, port down | Speed mismatch (10G module in a 1G port), or module rejected by coding |
| Port flaps up and down | Dirty connector, or a distance beyond the optic budget |
| Link up, errors and retransmits | Multimode optic on single-mode fibre or the reverse, or a bad patch lead |
| One direction works only | TX and RX swapped, or a broken strand in the patch lead |
| Worked on the bench, not in the rack | The installed fibre is a different type or longer than assumed |
| Amber port light | Module recognised but the far end is not configured or not powered |
Two field habits worth having: clean every connector before inserting it, because dust is the most common cause of a marginal fibre link, and keep one spare module of each type on site so a swap test takes a minute instead of a week.
Send the two switch models and we will confirm the part
Switch model at each end, link speed, fibre type and the real cable distance. We confirm the exact module, quote the pair, and tell you if a DAC would be cheaper.
Questions we get asked
What does SFP stand for?
Small Form-factor Pluggable. It is a hot-swappable module that slots into a switch, router or server port and converts the electrical signal into light for fibre, or into a long copper link. SFP+ is the 10 Gbps version of the same physical form factor.
Can I use an SFP module in an SFP+ port?
Usually yes, and most SFP+ ports accept a 1 Gbps SFP module and run that link at 1 Gbps. It is not universal: some switches, particularly certain Cisco and higher-end models, restrict the port or need the speed set manually. Check the switch documentation, and never assume it works in both directions.
Can I use an SFP+ module in an SFP port?
No. A 1 Gbps SFP port cannot drive a 10 Gbps module. The module may physically fit, because the form factor is identical, but the port will not run it. This asymmetry catches people out constantly.
Are SFP and SFP+ the same size?
Yes, physically identical, which is precisely why the mistake above is so common. The difference is electrical speed, not shape. SFP28 (25 Gbps) is also the same size; QSFP and QSFP+ are larger four-lane modules for 40 and 100 Gbps.
What is the difference between multimode and single-mode fibre?
Multimode has a wider core and uses cheaper optics over shorter distances, typically up to 300 to 550 m; the patch cables are usually aqua or orange. Single-mode has a narrow core and carries light much further, up to tens of kilometres, with more expensive optics; the cables are usually yellow. You cannot mix them: the module type must match the fibre already installed in the building.
Do I need two modules or one?
Two, one at each end, and they must be a matching pair of the same type and wavelength. The only exception is a BiDi (bidirectional) pair, which uses one fibre strand with an upstream and a downstream module that must be ordered as a matched set.
Will a third-party or compatible SFP work in my switch?
Often, yes. Modules carry a small chip with a vendor code, and switches may reject an unrecognised code or simply warn about it; on Cisco, an unsupported module can be enabled with a service command that also affects your support position. Third-party modules coded for your switch brand are widely used and generally reliable. What we will not do is sell you a module coded for the wrong brand and leave you to find out on site, so tell us the exact switch model when you order.
What is a DAC cable?
Direct Attach Copper: a fixed-length twinax cable with transceiver ends moulded on, for short links inside a rack, typically 1 to 7 m. It costs far less than two transceivers plus a fibre patch lead, uses less power and is the normal way to connect a switch to a server or to stack two switches in the same rack.
How far can each type go?
As a working guide: SR multimode 300 to 400 m, LR single-mode 10 km, ER 40 km, ZR 80 km, and copper SFP (RJ45) 100 m. Real distances depend on the fibre grade and the number of joints, so for anything close to the limit, plan a margin or have the link budget checked.
Do you stock SFPs in Dubai?
Yes, common Ubiquiti, Cisco and HPE modules are held or available quickly from official UAE distribution, brand new and sealed. Send us the switch model at each end and the fibre type and we will confirm the exact part before you order.
Written by the IRIX networking team in Bur Dubai. Browse SFP modules, switches and cables and connectors. Modules are brand new and sealed from official UAE distribution.