Optical transceivers — SFP, SFP+, QSFP, and friends

A transceiver is the swappable module that converts electrical signals from the switch into optical signals on the fiber (and back). The form factor and standard determine speed, reach, and whether the switch will accept it at all.

Form factors at a glance

| Form factor | Lanes | Speed range | Where you see it | |---|---|---|---| | SFP | 1 | 100 Mbps – 1 Gbps | Legacy 1 GbE, GPON, some serial | | SFP+ | 1 | 10 Gbps | The 10 GbE workhorse — everywhere | | SFP28 | 1 | 25 Gbps | DC server NICs, 25 GbE switches | | QSFP+ | 4 | 40 Gbps total | Switch uplinks, breakout to 4×10G | | QSFP28 | 4 | 100 Gbps total | Modern DC spines, breakout to 4×25G | | QSFP-DD / OSFP | 8 | 400 Gbps total | Hyperscale DC, breakout to 8×50G or 4×100G | | CFP / CFP2 / CFP4 | varies | 100 / 200 / 400 Gbps | DWDM client side, coherent modules |

SFP and SFP+ are mechanically identical (same cage); SFP28 also fits, just runs faster. Switches advertise which they accept — putting a 10G optic in a 1G-only port works at 1G; the reverse doesn't.

Naming the optic itself

Standards encode the reach in the suffix. The pattern is <speed>BASE-<reach><coding>:

| Suffix letter | Meaning | Typical reach | |---|---|---| | T | Twisted pair (copper) | 100 m | | SX | MMF short reach (850 nm) | 300 m on OM3 | | LX / LR | SMF long reach (1310 nm) | 10 km | | ER | SMF extended reach (1550 nm) | 40 km | | ZR | SMF zone reach (1550 nm, coherent on QSFP-DD) | 80–120 km | | BX | Bidi — single-fiber, two wavelengths | 10 km | | CWDM / DWDM | Tunable wavelength for WDM systems | varies |

Examples you'll see daily: 1000BASE-LX (1G, 10 km SMF), 10GBASE-SR (10G, MMF), 10GBASE-LR (10G, 10 km SMF), 100GBASE-LR4 (100G, 4 wavelengths on a single SMF pair, 10 km).

DDM / DOM — the optic talks back

Every modern transceiver supports Digital Diagnostic Monitoring. The switch can read out from the optic itself:

• Tx power (dBm)
• Rx power (dBm)
• Temperature (°C)
• Voltage and bias current
• Vendor, part number, serial, wavelength

On Cisco IOS: show interfaces transceiver. On Nokia SR OS: show port detail. On WeOS: show interface optical-monitoring. If Rx Power = -40 dBm (or "below sensitivity") you have no light — broken fiber, dirty connector, or far-end optic dead. Diagnose by comparing the Tx and Rx values you'd expect from the optic's datasheet.

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"Vendor-coded" optics — the perpetual headache

Some vendors enforce that only their branded optics will work — even when the underlying part is identical to a third-party module. Cisco does this aggressively; Nokia and Westermo are less strict; most whitebox vendors are open. Workarounds:

1. Buy genuine optics from your switch vendor. Most expensive.
2. Buy third-party optics from a reputable reseller and ask for them coded to your switch vendor's profile. Standard practice. Same hardware inside.
3. Disable optic check on the switch (service unsupported-transceiver on Cisco, mostly used in labs).

CCNA won't test you on vendor lock-in, but you'll meet it on day one of any procurement project.

What to remember

• Form factor (SFP / SFP+ / QSFP / QSFP-DD) sets the cage; standard (xBASE-X) sets the reach.
• Always read DDM before troubleshooting an optical link. The optic tells you exactly what's wrong.
• Vendor-coded optics are a procurement, not a technology, problem.