How it works

A Direct Attach Copper cable (DAC) is twinax copper with SFP+, SFP28, or QSFP cages molded onto both ends, so two switches or a NIC and a switch can run a short high-speed Ethernet link without buying separate optical transceivers. Passive DAC is the common rack pick: the hosts’ cages drive the copper directly, power draw stays low, and latency is tiny. Active DAC adds electronics in the cable for slightly longer copper runs. Neither is a fiber AOC (active optical cable); AOCs still look like DAC form-factor but carry light inside a thin fiber jacket for longer aisles.

The cages follow the same MSA footprint as hot-pluggable SFP+ optics, so you seat a DAC the same way you seat a module, except the “module” is permanently attached to the twinax. Typical 10G SFP+ passive lengths are 0.5 to 3 m for top-of-rack or adjacent-rack jumps; longer copper usually means thicker gauge, higher power, or a move to optics. Match speed and form factor on both ends (10G SFP+ to 10G SFP+, 25G SFP28 to SFP28, and so on). Vendor EEPROM coding can matter on picky switches; MSA “open” or Cisco-coded third-party DACs cover most home-lab and SMB gear.

Think of DAC as an Ethernet patch cord that already includes the pluggable ends, built for the short hops inside a rack instead of across a building.

When it matters

It matters when two SFP+/QSFP ports sit within a few meters and you want 10G (or faster) without the cost and optics inventory of fiber modules, not when the run crosses rooms or floors. Prefer DAC for switch stacking, NIC-to-ToR, or lab benches where a short twinax is cleaner than RJ45 multi-gig adapters. Skip DAC when you need longer reach, electrical isolation, or the hosts only expose copper RJ45, then use fiber optics, AOC, or a multi-gig copper path instead.

DAC does not replace link aggregation: if you need more aggregate bandwidth or failover across several ports, pair short DAC (or copper) members with LACP. SFP+ cages and 2.5/5/10G copper are related glossary topics when you are choosing between DAC and RJ45 multi-gig uplinks.

A short Cable Matters SFP+ DAC for adjacent ports, plus a 3 m 10Gtek twinax when the jump spans a bit more of the rack.

The Cable Matters 10Gbps DAC Twinax SFP+ Cable (0.5 m) is a passive 10GBASE-CU twinax for very short switch-to-switch or NIC-to-switch hops. View on Amazon when the ports sit in the same rack unit stack or neighboring chassis.

The 10Gtek SFP+ DAC Twinax Cable (3 m) is a passive 10G Direct Attach copper patch aimed at Intel-coded and many open SFP+ ports. View on Amazon when you need a longer in-rack twinax without moving to fiber optics.

For bundling multiple Ethernet members for bandwidth and failover, see LACP. For copper category cabling on RJ45 paths, see Cat6.