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All about Cisco Direct Attach Cables

When you need to do multi-gigabit networking, you need high-quality cables to carry the signal. While many people assume that fiber optics are necessary for these connections, there are cases where you can still use cable, and when you can, it saves a lot of money. If you’re still shopping for cables, it’s worth considering DAC as an option.

What Is DAC?

Direct Attach Copper (DAC) is a type of cable that enables high-speed connections over short distances. Specifically, these cables use SFP-style adapters to connect to a switch, transceiver, and other networking devices. Typically, they are fixed-length cables, and they are ideal for intra-switch connections at multi-gigabit speeds.

Specifications

Most commonly, DAC cables connect via SFP+, QSFP+, SFP28, QSFP56, and QSFP28. The maximum reliable distance for a DAC cable is 10 meters, and that’s for SFP+ connections. At the very highest speeds, 3-meter connection limits are common.

Speaking of speeds, DAC cables support 10G, 25G, and 40G connections.

When and Where to Use DAC

Knowing about DAC is one thing, but when is it best suited for a particular network? In general, it’s ideal for connecting SFP ports on your hardware, but even then, you’ll have a few options. The best way to review DAC use cases is to compare DAC to AOC. Then, we can review the differences between active and passive DAC.

DAC vs AOC

One common contention is when it is better to use DAC or AOC connections. AOC (active optical cable) is a fiber-optic cable that serves much of the same purpose as DAC. Both cable types connect devices via SFP and QSFP connectors, and both support multi-gigabit speeds.

The primary difference is that AOC can service much longer runs. While DAC caps at 10 meters, AOC can function nominally at distances up to 100 meters.

So, which should you use?

If DAC is viable, it’s the better choice as it typically costs much less than AOC. If you need longer runs or connection speeds beyond 40G, then AOC is the best choice and can serve your network well.

Active vs Passive

If you are using DAC, then you’ll have another choice before you: active vs passive cables. As the name suggests, active DAC cables support signal conditioning while passive cables do not.

Which is right for your network depends on the hardware you utilize. Active cables are necessary for ports that do not provide signal conditioning. Ports that do provide conditioning are better supported by passive cables.

Generally speaking, passive cables are less expensive, but hardware that enables passive DAC may cost more.

Additional Learning Center Resources