QSFP28 to SFP+ Breakout Notes: One 100G Port, Four Links

A 100G switch port is a powerful thing, and sometimes it is too powerful for the job. A row of servers, a set of access switches or a group of storage nodes may each need 10G or 25G, not 100G, and giving each of them a full 100G port wastes capacity and budget. A breakout assembly solves that mismatch: it takes one 100G QSFP28 port and splits it into four lower-speed connections, each serving a separate device. This note explains how breakout works, when it makes sense, and what to confirm before ordering.

The mechanism follows directly from the construction of QSFP28. A QSFP28 module sends four lanes of 25G each, running in parallel over a multi-fiber connector, and those four lanes are independent. A breakout assembly takes each lane to a separate connector, so one port serves four devices. The most common form is QSFP28 to four SFP+ connections, where each far end serves a 10G device. The rate adaptation between 25G lanes and 10G devices is handled by the equipment and the assembly design, and it is the part of breakout that most needs confirmation, because not every combination works in every switch.

The first confirmation is whether the port supports breakout at all. Not every 100G port can be broken out: the port must be configured for breakout mode, and the switch documentation, not the cable catalogue, is the authority on that question. A buyer planning a breakout should check the switch documentation first, before ordering any assembly, because an assembly ordered for a port that does not support breakout is a wasted order. The check costs a page of documentation and saves a failed deployment.

The second confirmation is the far end. Each of the four far-end devices must match the lane it connects 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, their fiber types and their connectors during the inquiry, so each leg can be confirmed rather than assumed.

The third confirmation is the plant. A breakout assembly has a switch end and four far ends, and the cabling between them must be planned as five connections, not one. The switch end needs a multi-fiber connection; each far end needs its own connection, its own patch cord or cable, and its own panel port. A site that plans breakout as a single cable will find itself short of panel ports and patch cords at installation time. Planning the five connections together, with lengths measured along the actual routes, avoids that surprise.

When does breakout make sense, and when does it not? It makes sense where a 100G port would otherwise serve a device that needs only 10G or 25G, and where four such devices sit near the switch. It makes sense in migration, where a row is moving from 10G to 100G in stages and some devices are still at the lower speed. It does not make sense where all four far-end devices need 100G, because then the port should serve one 100G device directly. It does not make sense where the far-end devices are far apart, because the breakout assembly is a short assembly and the far ends must sit near each other. Breakout is a density and migration tool, not a general-purpose connection method.

The alternative to breakout, for a duplex plant, is a CWDM4 module, which multiplexes the four lanes onto different wavelengths over a single pair of fibers rather than splitting them to separate devices. The choice between breakout and CWDM4 is a choice about the plant: multi-fiber plant favours breakout and parallel optics, duplex plant favours CWDM4. A buyer should state the plant type during the inquiry, so the right option can be confirmed.

Before ordering a breakout assembly, confirm quantity, price and delivery time during the inquiry, together with the switch model and its breakout support, the four far-end devices and their speeds, fiber types and connectors, and the lengths along the actual routes. Certificates are available upon request. Where a specification is needed that this site does not publish, ask for it during the inquiry, and the answer will come from the published specification document rather than from an estimate.

Cable management for a breakout assembly deserves planning, because a breakout multiplies connections. One 100G port becomes five connections: one at the switch end and four at the far ends, each with its own patch cord or cable, its own panel port and its own label. A site that plans breakout as a single cable will find itself short of panel ports, short of patch cords and short of labels at installation time, and the resulting tangle will make future faults harder to find. Planning the five connections together — ports, cords, lengths and labels — turns a breakout from a tangle into a tidy installation, and the time spent planning is repaid at the first fault.

Failure domains are the second planning question for breakout, and they cut against its density advantage. When one 100G port serves four devices, a failure at that port — a faulty module, a damaged assembly, a configuration error — affects four devices at once, where four separate ports would have isolated the failure to one device each. That concentration is acceptable where the four devices are related, such as four servers in one rack, or where the alternative is four ports the switch does not have. It is less acceptable where the four devices are unrelated and critical, such as four separate services. A buyer planning breakout should ask, for each port, whether four devices sharing one failure domain is acceptable, and if it is not, the port should serve one device directly.

Cost comparison for breakout should be done honestly, because the headline saving is not the whole story. A breakout assembly costs less than four separate 100G ports and their modules, which is the saving that usually motivates the choice. Against that saving stand the costs of the four far-end connections, the panel ports, the patch cords, the labelling and the additional planning, plus the operational cost of a shared failure domain. In most cases breakout still wins, especially where switch ports are scarce, but a buyer who compares only the assembly price against four module prices will overstate the saving. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.

Documentation for a breakout installation is the practice that makes the other planning pay off. A record that states, for each 100G port, the four far-end devices, their speeds, their fiber types, their connectors and the lengths along the actual routes turns future fault-finding from detective work into routine work. Without that record, a fault on one leg of a breakout requires tracing four legs to find the one that failed, and a crew working under pressure will waste time on the three healthy legs before reaching the faulty one. With the record, the faulty leg is identified from the error counters and the record together, and the repair is a single leg, not an investigation of all four.