For links between switches and servers you have three choices: a direct-attach copper cable (DAC), an active optical cable (AOC), or two transceivers joined by a fibre patch cord. Each has its place. This guide helps you pick quickly.
The short answer
- Same rack, up to about 3 m: use a DAC. It is the cheapest option and uses the least power.
- Neighbouring racks, up to about 30 m: use an AOC. It is thin and light, and reaches further than copper.
- Longer runs, structured cabling or patch panels: use optics plus fibre.
Side by side
| DAC | AOC | Optics + fibre | |
|---|---|---|---|
| Typical reach | up to 3–5 m (passive), ~7–10 m (active) | up to ~30 m, some to 100 m | 100 m to 80 km+ |
| Cost per link | lowest | low to medium | highest (two optics + cable) |
| Power | lowest (passive ≈ 0 W) | low | higher |
| Cable thickness | thick and stiff | thin and flexible | thin; any length |
| Works through patch panels | no | no | yes |
| Replace one end only | no, the whole cable | no, the whole cable | yes |
| Latency | lowest | low | low |
When to choose DAC
- Server to top-of-rack switch in the same rack
- Switch-to-switch stacking or MLAG links within a rack
- When cost and power matter most
Watch out for:
- Bend radius and bulk: thick twinax cable can block airflow in dense racks.
- Length: passive DACs are reliable up to about 3 m at 25G and 100G; longer runs may need active DACs or FEC enabled.
- Breakout: a breakout DAC (for example QSFP28 to 4 × SFP28) splits one fast port into four slower ones. Configure breakout mode on the switch first.
When to choose AOC
- Rack-to-rack links within a row
- Dense cabling where thick copper would block airflow
- When you want a plug-and-play link without cleaning and inspecting fibre connectors
Watch out for:
- Fixed length: order the right length, because spare cable has to be coiled.
- No patch panels: the cable is one piece from end to end.
When to choose optics and fibre
- Any run longer than an AOC can reach
- Links that cross patch panels or structured cabling
- Campus and long-haul links (LR, ER, ZR)
- When you want to reuse fibre already installed, or replace one optic without re-cabling
Watch out for:
- Matching: both optics must use the same standard (SR to SR, LR to LR).
- Fibre type: multimode (OM3/OM4) for SR, single-mode (OS2) for LR and beyond.
- Clean connectors: dirty connectors are the most common cause of link errors.
Speeds and form factors
- 10G: SFP+ DAC, SFP+ AOC, or SFP+ optics
- 25G: SFP28 DAC or AOC, or SFP28 optics
- 40G: QSFP+ DAC or AOC, including 4 × 10G breakout
- 100G: QSFP28 DAC or AOC, including 4 × 25G breakout
- 400G: QSFP-DD DAC or AOC, including breakout options
Compatibility
DACs and AOCs carry an identification chip, just like optics. Use the brand that matches your switches: for example, a Cisco DAC between two Cisco switches. When connecting different brands, check that both platforms support the cable.