QSFP28 Breakout Guide

A QSFP28 breakout cable or module splits one 100G port into four lower-speed connections, and it exists because the four-lane construction of QSFP28 makes it possible. A QSFP28 module sends four lanes of 25G each, and those four lanes are independent. A breakout assembly takes each lane to a separate connector, so one 100G port serves four 25G devices, or, with the right assembly, four 10G devices through rate adaptation. That is the whole idea, and it is what makes mixed-speed designs practical without replacing the switch.

The most common breakout is QSFP28 to four SFP+ connections. The switch end carries a QSFP28 connector, and the far end carries four SFP+ connectors, each serving a 10G device. The rate adaptation between 25G lanes and 10G devices is handled by the equipment and the assembly design, and a buyer planning such a breakout should confirm that the switch supports the breakout mode, because not every 100G port can be broken out: the port must be configured for breakout, and the switch documentation, not the cable catalogue, is the authority on that question.

The CWDM4 variant is a different use of the same idea. Where a breakout splits lanes to separate devices, a CWDM4 module multiplexes the four lanes onto different wavelengths over a single pair of fibers, carrying 100G over a duplex connection rather than a multi-fiber one. That suits links where the fiber plant is duplex rather than multi-fiber, or where the distance favours wavelength multiplexing over parallel fibers. The choice between a breakout assembly and a CWDM4 module is therefore a choice about the plant: multi-fiber plant favours breakout and parallel optics, duplex plant favours CWDM4.

Planning a breakout needs the same discipline as planning any link, with one extra question. The four far-end devices must each match the lane they connect to, in speed, fiber type and connector. A breakout that serves four devices with different fiber types or different speeds needs each leg confirmed separately, because the breakout assembly does not convert between them. A buyer should list the four far-end devices, their speeds and their connectors during the inquiry, so each leg can be confirmed.

This guide states breakout practice at category level. No breakout cable or module specification is stated here as a product claim, because none has been published for a breakout product. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.