How to choose the right SFP module

1. Form factor, will it physically fit?

The form factor is the shape of the module and the cage it slots into. A module will only fit the cage it was built for; there is no adapter that turns an SFP+ cage into a QSFP cage.

Form factor	Typical speed
SFP	1G, access switches, older uplinks
SFP+	10G, the workhorse of most networks
SFP28	25G, modern server access
QSFP+	40G, switch-to-switch uplinks
QSFP28	100G, spine and data-centre core
XFP	10G, older 10G platforms

An SFP+ module will usually drop into an SFP cage and run at 1G, but a 10G-only module will not negotiate down on every platform. Check the switch datasheet if you are mixing rates.

2. Speed, match both ends

Both ends of a link must run the same rate. A 10G module will not talk to a 1G module. Where a switch port supports several rates, the slower end normally dictates the link speed, but only if both modules support that rate.

3. Connector, fibre or copper?

• LC, the standard duplex fibre connector, two strands, one to transmit and one to receive. This is what most optical modules use.
• RJ45, copper Ethernet over Cat5e or Cat6, up to 100 m. Convenient, but it draws more power and adds latency compared with fibre.
• MPO/MTP, a multi-fibre ribbon connector used by 40G SR4 and 100G SR4 modules, which run four parallel lanes.
• DAC, a direct attach copper cable with the transceivers moulded on both ends. The cheapest way to link two switches inside the same rack.

4. Distance, SR, LR, ER and ZR

The reach class tells you how far the light will carry and which fibre type you need. Putting an SR module on single-mode fibre, or an LR module on multimode, is one of the most common reasons a link fails to come up.

Class	Fibre & reach
SR (short reach)	Multimode, 850 nm, up to 300 to 550 m
LR (long reach)	Single-mode, 1310 nm, up to ~10 km
ER (extended reach)	Single-mode, 1550 nm, up to ~40 km
ZR (ultra long haul)	Single-mode, 1550 nm, up to ~80 km

Multimode fibre is usually orange or aqua; single-mode is usually yellow. If the fibre is already installed, that colour decides your module, not the other way round.

5. Vendor coding, the one that catches people out

Every transceiver carries an EEPROM that identifies it to the switch. Cisco switches look for Cisco coding, Huawei for Huawei, and so on. A module that is perfect on every spec above will still sit dark if it is coded for the wrong vendor.

This is why we ask which switch you are fitting the module into. We match the part number and its factory coding to the platform you named, so it comes up as a supported optic rather than throwing an unsupported-transceiver error. Refurbished stock is also powered up and link-tested in matching hardware before it ships.

Shop by what you now know

1G SFP · 10G SFP+ · 25G SFP28 · 40G QSFP+ · 100G QSFP28
Short reach (SR) · Long reach (LR) · Copper RJ45 · DAC cables