Choosing optics and cables for a 25G/100G build
DAC, AOC or optics — how to pick the right physical link, and why optics belong in the first quotation.
On a 25G/100G data centre build, optics and cables commonly run to 30–40% of the hardware value. They are often left until the end of a project and then delay it. Choose them with the switches.
Start with distance
| Distance | First choice | Notes |
|---|---|---|
| Inside one rack | Passive copper (DAC) | Cheapest, lowest power. 3 m covers a 42U rack with routing slack. |
| Adjacent racks | DAC up to 5 m, then AOC | Passive copper tops out at 5 m for 25G and 100G (10 m for 10G). |
| Same room, ≤100 m | Short-reach optics on OM4 | 25G-SR reaches 100 m on OM4 but only 70 m on OM3. |
| Between buildings, ≤10 km | Long-reach optics on single-mode | LR optics; plan patch panels and spare fibre pairs. |
Three traps to avoid
- Reusing old multimode fibre. 10G-SR ran 300 m over OM3; 25G-SR does not. On existing OM3 plant, use extended-reach (CSR) optics or plan new OM4.
- Assuming breakout works everywhere. Splitting a 100G port into 4 × 25G is supported on some switch models and ports and not others. Check the vendor's compatibility matrix for the exact model, port and software release.
- Ordering the wrong airflow. Fans and power supplies come as port-side intake or port-side exhaust. It is a different part number, not a setting. Match it to your hot and cold aisles.
Spares
Add 5–10% spare optics and at least one spare of each cable length. A single failed optic should never wait on a shipment.