Choosing an optical transceiver: speed, reach, fibre and connector

Most optics orders that go wrong go wrong for the same handful of reasons: the right speed but the wrong fibre, the right standard but the wrong connector, or a module that physically won't seat in the cage. Four decisions settle it, and they happen in this order.

1. The port decides the form factor

Form factor is the physical shape of the module and the cage it slides into. It tracks speed closely:

• SFP — 1G
• SFP+ — 10G
• SFP28 — 25G
• SFP56 — 50G
• QSFP+ — 40G
• QSFP28 — 100G
• QSFP56 — 200G
• QSFP-DD and OSFP — 400G and 800G

Cages are usually backward compatible within a family: an SFP+ port will generally take a 1G SFP, and a QSFP28 port will generally take a 40G QSFP+. Generally, not always — some platforms restrict it in software or by line card, so check the switch documentation before you plan around it.

QSFP-DD and OSFP are the one place people get caught out. They are different cages, not two names for the same thing, and a QSFP-DD module will not seat in an OSFP port. At 400G and 800G, confirm which one your line card uses before anything else.

2. The fibre you have decides the standard

Multimode is cheap over short distances and runs out of reach quickly. Single-mode costs more per link but goes as far as you need. The optical standard in the part number tells you which one it expects:

• Multimode — SR, SR4, SR8, VR, CSR4, SR-BiDi. Typically 70–100m on OM3 and 100–150m on OM4, with 10G SR reaching 300m.
• Single-mode — DR (500m), FR (2km), LR and LR4 (10km), ER and ER4 (40km), ZR (80km).

If you are unsure what is in the ground, the jacket colour is a reasonable first clue — aqua or lime for OM3/OM4 multimode, yellow for single-mode — but the only reliable answer is the cable's own printed marking.

3. The standard decides the connector

This is the step people skip, and it is the most expensive one to get wrong, because the module is fine and the trunk on the floor is not.

• Duplex LC — two fibres, one transmit and one receive. SR, LR, ER, ZR, FR4, LR4, CWDM4.
• Simplex LC — a single fibre carrying both directions on different wavelengths. Every BX and BiDi part.
• MPO-12 — a 12-fibre ribbon trunk. SR4, DR4, PSM4, and 100G parallel optics generally.
• MPO-16 — 16 fibres, used by 400G SR8 and DR8 class parts.
• RJ-45 — copper twisted pair, 1000BASE-T and 10GBASE-T.

The classic mistake is assuming an SR4 takes an LC patch cord because an SR does. It does not: the 4 means four lanes, and four lanes need an MPO trunk. Our compatibility pages list the connector on every part, and so does the spec table on each product page.

4. The switch decides the coding

A transceiver carries a small EEPROM holding its identification data, and that is the only thing the switch reads to decide what has been plugged in. The optics are governed by the same MSA regardless of who built the module, but the identification is vendor-specific — which is why a module coded for Cisco is not the same part as one coded for Juniper even when the hardware inside is identical.

Coding means programming that EEPROM to present the module as the part the switch expects. Done properly, the switch brings it up exactly as it brings up the original: the port comes online, DOM reports transmit and receive power, and there is no service unsupported-transceiver command to add and no unsupported-transceiver warning in the log. A module that needs either of those has been coded badly, and that is the experience behind most of the folklore about third-party optics being trouble.

That is the whole of what we do. Every part is programmed for the specific platform it is sold for and tested on that platform before it ships, which is why the same physical module appears in our catalogue under a Cisco part number, a Juniper one, an Arista one and so on. Buy the one that matches your switch, not the one that matches the optics. There is more on what this means for support and warranty in a separate guide.

Putting it together

A 100G link between two spine switches 8km apart on single-mode fibre with LC patch cords is a QSFP28, LR4, duplex LC — a QSFP-100G-LR4-S for a Cisco platform. The same 100G link inside one row on existing OM4 with an MPO trunk already pulled is a QSFP-100G-SR4-S. Same speed, same switch, entirely different part, and the fibre in the floor is what decided it.