QSFP28 vs QSFP

QSFP28 and QSFP are two generations of the same quad form factor, and the difference between them is speed. QSFP, in its original form, delivers 40G through four lanes of 10G each. QSFP28 delivers 100G through four lanes of 25G each. The physical shape is closely related, and the two formats share the same design idea — four lanes in parallel in one module — but they are not the same product, and a buyer should not treat them as interchangeable without checking the equipment.

The lane construction explains both the similarity and the difference. A quad module sends four lanes in parallel over a multi-fiber connector, and the total speed is the lane speed multiplied by four. At 10G per lane, the total is 40G. At 25G per lane, the total is 100G. That is why a QSFP28 port can be broken out into four 25G connections, and why a QSFP port can be broken out into four 10G connections: the breakout follows the lanes. The breakout guide on this site explains both options.

Compatibility between the two formats is a question for the equipment, not for the module alone. Some 100G ports accept 40G modules and run at 40G; some do not, and the port will only accept the speed it was designed for. The switch documentation, not the module catalogue, is the authority on that question, and a buyer planning a mixed-speed design should check it before ordering. The same applies in reverse: a 40G port will not run a 100G module at 100G, because the port electronics set the ceiling.

The choice between 40G and 100G is a capacity and cost decision. A 40G link suits aggregation layers where many 10G links converge and where 100G would be over-provisioned. A 100G link suits core layers, data-centre interconnects and any link where traffic already approaches 40G or is expected to grow past it. The cost per gigabit falls as speed rises, but the absolute cost of a 100G port and its modules is higher, so the right choice is the lowest speed that comfortably carries the traffic with room to grow, not the highest speed available.

On this site, the fully specified 100G model is the QSFP28 LR4, and the fully specified 40G model is the QSFP+ SR4, each on its own detail page with its own published figures. The series pages for the two families list the remaining titles at series level. Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.