This guide provides a systematic selection process to help you choose the right QSFP28 module every time. You will learn how to verify form factor compatibility, match fiber and distance requirements, validate switch compatibility, consider thermal constraints, and. In March 2024, he installed a QSFP28 LR4 module in what he believed was a standard 100G port on his new switch. The module seated correctly and the latch engaged, but the link refused to come up. After three hours of troubleshooting, Marcus discovered the real issue. Refer to 400G Q-DD optical interoperability with slower speed optics in the QSFP-DD chapter for connecting 100G SR4 or SR2 optics to split 400G SR8 optics. 100G SR4 optics can be used by a QSFP28 port that can be "split". In this guide, we provide a comprehensive, practical overview of 100G QSFP28 modules, covering their working principles, module types, key specifications, typical applications, and a step-by-step selection framework to help you make confident, informed decisions for your network. What Is 100G. An engineer-focused, “just tell me what to choose” guide to transceiver selection with architecture, power budget, compatibility, and upgrade plan — designed for 25G/100G today and 400G/800G tomorrow. SFP-family modules are best for lower-speed edge and server links, QSFP-family modules serve higher-density aggregation and spine-leaf networks, and QSFP-DD is designed for 400G.