DAC, Optics or Copper: Cabling 10GbE

Once servers and switch agree on 10GbE, one question remains: what physically connects them? Three answers coexist — direct-attach copper, optical transceivers, and 10GBASE-T over twisted pair — and each is the obviously correct choice in its own territory. Trouble only arrives when one is used in another's.

DAC: the default inside a rack

Direct-attach copper — a fixed cable with SFP+ ends — is the cheapest, coolest and simplest 10/25GbE link that exists. No transceivers, negligible power (<0.5W), latency as low as it gets, nothing to configure. Passive DACs cover up to ∼5–7m: server to top-of-rack, done. The whole refurbished ecosystem — X520s, ConnectX cards, secondhand enterprise switches — treats DAC as the native tongue. Its only limits are reach and bend radius stiffness in dense arms.

Optics: when distance enters

SFP+ transceivers plus fiber take over past the rack: 10GBASE-SR on multimode (OM3/OM4) covers 300–400m — the between-rooms and between-floors answer — while LR on single-mode reaches 10km for the genuinely separated. Per-end cost with third-party optics is modest; power sits near 1W per module. The detail that ambushes first-timers: vendor coding. Many switches (and some NICs) reject optics or even DACs not flagged with their brand — the hardware is identical, the EEPROM string is the product. Third-party sellers code modules per vendor for the same price; just order Cisco-coded, HPE-coded, etc., to match each end, and keep one each of the majors in the spares drawer.

10GBASE-T: the installed-base answer

Ten gigabits over RJ45 and Cat6a (Cat6 for short runs) means reusing structured cabling and mixing with 1GbE gear seamlessly — genuinely valuable in offices already wired. The costs are physical: 2–4W per port at both ends (heat that multiplies across a switch), microseconds more latency from the heavier encoding, and pickier behavior over marginal old cable. For a handful of links across an existing building: perfect. For a fresh rack build: DAC beats it on every axis except familiarity.

The pattern that works

In-rack: DAC everywhere. Between racks/rooms: SR optics on OM4. Into the legacy cable plant or to 10GBASE-T-only devices: RJ45 where required, and only there. On refurbished NICs, prefer SFP+ models (X520/X710, ConnectX) over 10GBASE-T variants — cooler, cheaper links, and the flexibility to speak all three dialects as the network grows. Then label both ends, because future-you tracing a purple DAC through a loom at midnight deserves the kindness.