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Optics compatibility for AI fabrics

Six things have to agree before a link comes up. Get one wrong and you have a port that stays down for a reason nothing in the spec sheet explains.

Optics are where AI network bills of materials go wrong most often, because the failure is not obvious on paper. Every part number looks plausible, the speeds match, and the link does not come up.

The six axes

1. Form factor

The module has to physically fit the cage. OSFP will not fit QSFP-DD, and neither will fit QSFP112 or QSFP56. Switches and NICs often differ here even within one vendor's 400G line, so an 800G-capable OSFP switch paired with QSFP-DD NICs needs a different module at each end of the same link.

2. Vendor coding

Transceivers carry a vendor identity in EEPROM, and switches may refuse modules that are not coded for them. A Cisco-coded optic in an NVIDIA switch, or the reverse, is a supportability problem at best and a dead port at worst. Generic or universally-coded optics avoid this but change the support conversation.

3. Reach

A DAC is good for a couple of metres, an AOC for tens, SR for a hundred or so, DR and FR for hundreds to a couple of kilometres, LR beyond that. Reach has to cover the actual cable run: slack, riser and patch panels included, not the straight-line distance between racks.

4. Fibre type

Single-mode and multi-mode are not interchangeable. SR optics need OM3 or OM4 multi-mode; DR, FR and LR need OS2 single-mode. Mix them and the link either fails to come up or comes up and errors.

5. Fibre connector

MPO-12 will not patch to MPO-16. Eight-lane 800G parts commonly use dual MPO-12, some use MPO-16, and LC duplex appears on lower-lane parts. The trunk connector has to match the module connector at both ends, and a structured cabling plant standardised on one will not take the other without conversion.

6. Ferrule polish

APC will not mate with UPC. The angled physical contact ferrule and the ultra physical contact ferrule are mechanically incompatible, and forcing them damages both. This is the axis most often left out of a compatibility check altogether, because it does not appear in the speed or the form factor and lives only in the connector part number.

The seventh: heatsink form

OSFP modules come flat-top and finned-top. A finned-top module will not fit a switch cage designed for flat-top optics with its own riding heatsink. There is no electrical incompatibility, so nothing in a specification comparison flags it. The module simply will not seat.

Twin-port modules and how many to order

A twin-port 800G module terminates two 400G links, so it counts as half a module per link. Switches like the QM9700 present 64 logical 400G ports through 32 physical cages for exactly this reason. One module per logical port doubles the optic spend. One per cage without accounting for the split miscounts the links.

Cheapest media that actually works

For a given link the sensible order of preference is structured copper, then DAC, then AOC, then transceivers plus fibre. Cheapest first, subject to reach and to every axis above. DAC and AOC have no separate transceivers to count. A fibre link has two.

Verify Cisco parts against the source. Cisco publishes the Transceiver Module Group (TMG) matrix, which is authoritative for which optics a given platform supports. This tool's Cisco optic entries are checked against it automatically, not transcribed by hand.

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