Breakout Cables Explained: 400G to 4x100G, 100G to 4x25G and More

A breakout cable splits one high-speed switch port into several lower-speed links, for example one 400G port into four 100G links or one 100G port into four 25G links. It works because the faster port is already built from parallel electrical lanes, and the breakout simply gives each lane (or group of lanes) its own connector at the far end.

Breakouts are a cheap way to get more usable ports from a switch or run 25G servers from 100G ports. They also cause a lot of “link down” tickets when lane speeds, port mode or optics do not match. This guide covers the common mappings, copper vs optical breakouts, and what to check before you order.

How a Breakout Cable Works

Every QSFP-family port is a bundle of lanes. A 40G QSFP+ port is 4 x 10G, a 100G QSFP28 port is 4 x 25G, a 400G QSFP-DD or OSFP port is 8 x 50G PAM4 (or 4 x 100G PAM4 on QSFP112), and an 800G port is 8 x 100G PAM4. When you put the port into breakout mode, the switch treats each lane or pair of lanes as its own interface, with its own MAC, counters and configuration.

A 100G to 4 x 25G breakout gives you four logical ports, often named something like Ethernet1/49/1 to 1/49/4, each at 25G. Total bandwidth stays the same.

The key rule: a breakout cannot change the lane rate. If the big port runs 50G per lane, each leg must also run 50G per lane. This explains most failed breakout designs.

Common Breakout Mappings

The table below lists the mappings you will meet most often in enterprise and data center networks. Lane rates are electrical lanes at the host interface.

8-way splits such as 400G to 8 x 50G also exist and follow the same lane logic.

DAC and AOC Breakouts vs Optical Breakout

You can buy a breakout as one fixed assembly (copper or active optical), or build it from a parallel optic, a fiber harness and separate optics at the far end.

DAC breakout cables

A direct attach copper (DAC) breakout is a single twinax assembly with one QSFP plug on one end and two, four or eight smaller plugs on the other. It uses almost no power and is the lowest-cost option, but reach is short. At 10G per lane, passive copper breakouts commonly run to 5 metres. At 25G per lane, expect about 3 metres for most use, with 5 metre lengths usually needing RS-FEC on both ends. At 50G and 100G PAM4 per lane, passive lengths shrink further, typically 2 to 3 metres at 50G and around 2 metres or less at 100G per lane. DAC breakouts suit top-of-rack switch to servers in the same rack.

Longer copper runs at high lane rates are covered by active electrical cables (AECs), which add retimers in the plugs. You can see the options under active copper and AEC cables.

AOC breakout cables

An active optical cable (AOC) breakout has optics sealed into each plug and fiber in between. It is lighter than copper, reaches tens of metres and has no connectors to clean, which suits links between adjacent racks. Like a DAC, its length is fixed.

Optical breakout with transceivers

For longer or structured runs, use a parallel optic at the big end and separate transceivers at the far end:

• Single-mode DR4 to 4 x DR: a 400G-DR4 module (4 x 100G PAM4 on 4 fiber pairs, MPO-12, 500m on single-mode) breaks out to four 100G-DR QSFP28 modules with LC duplex connectors. 400G-XDR4 (also sold as DR4+) extends this to 2km and pairs with 100G-FR modules.
• Multimode SR4 MPO-to-LC harness: a 40GBASE-SR4 or 100GBASE-SR4 module (MPO-12, 8 fibers used) breaks out to four 10GBASE-SR or 25GBASE-SR modules through an MPO to 4 x LC duplex harness or a breakout cassette. 100GBASE-SR4 reaches 70m on OM3 and 100m on OM4.

At 800G the same idea is built into the module: an 800G-2xDR4 or 800G-2xFR4 already presents as two independent 400G links on two connectors, and 800G-DR8 can split into eight 100G-DR links.

Optical breakouts cost more than a DAC, but they work with structured cabling and let you mix lengths. Our MTP/MPO trunks and harnesses cover the fiber side. Polarity matters with harnesses, so check the MTP/MPO polarity guide before you order.

Lane and Speed Matching: Where Breakouts Go Wrong

Most breakout failures come from a lane mismatch, not a bad cable. Both ends must agree on lane count, lane rate, modulation and FEC.

100G is not always 4 x 25G

A 100GBASE-SR4 or 100G-CWDM4 module uses four 25G NRZ lanes. A 100G-DR or 100G-FR module uses a single 100G PAM4 wavelength on one fiber pair, with a gearbox converting the 4 x 25G electrical interface inside the module. A 100G QSFP56 port may run 2 x 50G PAM4. All are “100G”, but they are not interchangeable.

So a 400G-DR4 port breaks out to 100G-DR modules, not to 100G-SR4 or 100G-CWDM4. And a 400G QSFP-DD to 4 x 100G DAC whose legs run 2 x 50G PAM4 will not light up in a server NIC that only supports QSFP28 at 4 x 25G NRZ. If your hosts are QSFP28-only, use 400G-DR4 to 100G-DR optics, or check that the DAC is specifically built for that pairing.

FEC and auto-negotiation

25G links usually need RS-FEC (Clause 91/108) with SR optics and longer DACs, while some short DACs run with FC-FEC or no FEC. PAM4 links at 50G and 100G per lane always use RS-FEC. If one end uses RS-FEC and the other uses none, the link stays down. Set FEC explicitly on both ends rather than relying on defaults, especially when one end is a switch and the other is a server NIC.

Configuring Breakout Mode on the Switch

The switch port must be put into the matching breakout mode, and the syntax differs by vendor. On Cisco Nexus it is an interface breakout command, on Arista EOS it is usually set with a speed command on the first sub-interface, on Juniper it is a channel-speed or port-profile setting, and SONiC uses a breakout mode such as 4x25G.

Check the switch documentation for:

• Which ports support breakout. Some platforms only allow it on certain ports, or only on odd-numbered ports.
• Port group limits. Breaking out one port can disable a neighbouring port, because the ASIC has a fixed number of logical ports per group.
• Supported modes. A 400G port might support 4 x 100G and 2 x 200G but not 8 x 50G.
• Disruption. Changing breakout mode often flaps the port, and some platforms need a reload. Plan it in a maintenance window.

Optics and cables also need to be coded for the switch. TIFNEX codes breakout DACs, AOCs and transceivers for Cisco, Juniper, Arista, Dell, HPE, NVIDIA/Mellanox and other platforms, and tests them before shipping.

Reach Limits by Breakout Type

Copper figures are typical passive lengths; optical reaches are the IEEE or MSA specification.

Treat the copper rows as guidance and confirm against the switch vendor’s supported cable list.

How to Choose the Right Breakout

Start from the far end, not the switch. What port does the server, storage array or downstream switch have, and what lane rate does it run? Then pick the medium by distance: DAC inside the rack, AOC or AEC to the next rack or two, and transceivers with structured fiber beyond that. For a deeper comparison of the three, see DAC vs AOC vs transceivers.

Checklist before ordering:

1. Switch model, port type (QSFP28, QSFP56, QSFP-DD, OSFP) and confirmation that the port supports the breakout mode you need.
2. Far-end port type and lane rate (for example SFP28 at 25G NRZ, or QSFP28 100G-DR).
3. Distance per leg, including routing through trays and patch panels.
4. FEC requirement on both ends.
5. Platform coding for each end if the two ends are different vendors.
6. For optical breakouts: fiber type (OS2, OM3, OM4), MPO polarity, and whether a harness or a cassette fits your panel layout.
7. Spare legs or spare cables for each rack.

Browse breakout cables, 100G QSFP28 transceivers and 400G transceivers or let our engineers match parts to your switch.

Frequently Asked Questions

What is a breakout cable used for?

A breakout cable splits one high-speed switch port into several lower-speed links, such as one 100G QSFP28 port into four 25G SFP28 links. It is used to connect servers or older switches to faster ports and to make full use of switch port capacity.

Do I need to configure the switch to use a breakout cable?

Yes. The switch port must be set to the matching breakout mode, such as 4x25G or 4x100G, before the individual links come up. Check that the port supports that mode, because some platforms limit breakout to certain ports or disable a neighbouring port.

Can a 400G port break out to standard 100G QSFP28 optics?

Yes, if you use the right optics. A 400G-DR4 module breaks out to four 100G-DR QSFP28 modules over single-mode fiber, but it will not work with 100G-SR4 or 100G-CWDM4, because those use four 25G lanes instead of one 100G lane.

How long can a DAC breakout cable be?

It depends on the lane rate. A 40G to 4 x 10G passive DAC typically runs up to 5 metres, a 100G to 4 x 25G DAC about 3 metres (5 metres with RS-FEC), and 50G or 100G PAM4 lane breakouts around 2 to 3 metres or less. For longer runs use an AOC, an active electrical cable or optics.

What is the difference between a breakout harness and a breakout cassette?

A harness is a cable with an MPO connector on one end and several LC duplex connectors on the other, used for direct connections. A cassette does the same conversion inside a patch panel module, which keeps structured cabling tidy and makes moves and changes easier.

Talk to TIFNEX

Send us your switch model, the far-end NIC or switch ports, the distance for each leg and the quantities you need, and we will come back with a matched breakout, DAC, AOC or optics option within 24 hours. TIFNEX delivers across the UAE, Egypt and the wider GCC, and our engineers can confirm port modes and coding before you order. Contact our team or WhatsApp us on +971 54 300 8970.