A four-lane QSFP running at 100G per lane — 400G in the familiar QSFP footprint.
What Is QSFP112?
QSFP112 is a four-lane QSFP where each lane runs at 112 gigabits of signalling — 100G of payload after encoding overhead. Four times 100G is 400G, delivered in a single-row QSFP body rather than the double-density body QSFP-DD needs.
It exists because per-lane speed caught up. When 400G first shipped, 100G per lane was not manufacturable, so the industry got there by widening to eight lanes at 50G. Once 112G SerDes matured, the narrower, simpler option became available.
Why Fewer Lanes Is Better
Fewer, faster lanes is generally the better engineering answer: fewer optical components, fewer fibres in the cable, less power per bit. A 400G QSFP112 DR4 uses four fibre pairs where a 400G QSFP-DD SR8 uses eight, which halves the structured cabling requirement for the same bandwidth.
Across a large build that difference is the dominant cost, not the module price. Work it through in the cabling planner with your own port count.
The Constraint
A switch can only offer QSFP112 if its SerDes run at 112G, which means a newer ASIC generation. On an older platform, eight lanes at 56G signalling is the only way to reach 400G, whatever cage is fitted.
This is worth asking about explicitly when evaluating platforms, because the answer determines which form factors are available to you for the life of the switch — and therefore how much fibre a given amount of bandwidth will consume.
Where You Meet It
On 51.2T-class switches offering 400G ports, and as the host-side module in 800G-to-2×400G breakout designs. The breakout configurator works out what a given port needs on the fibre side.
QSFP112 reference
| Lanes | 4 |
|---|---|
| Per-lane rate | 112G signalling, 100G payload |
| Port speed | 400G |
| Host requirement | 112G SerDes |
| Typical use | 400G ports on 51.2T switches; host side of 800G breakout |
| Fibre for DR4 | 8 fibres — half an SR8 build |