Gigabit optical ports can be used with 100Mbps modules

Yes, a Gigabit Ethernet optical port can operate at 100Mbps using a specialized 100BASE-FX SFP module like Cisco GLC-GE-100FX.How It WorksStandard Gigabit SFP modules (1000BASE-SX/LX) are fixed at 1 Gbps and cannot natively run at 100 Mbps. To connect a legacy 100 Mbps fiber link to a modern Gigabit Ethernet port, you need a 100BASE-FX SFP module designed for this purpose. The Cisco GLC-GE-100FX module, for example, presents a 1 Gbps SGMII interface to the host port while converting the signal to 100 Mbps Fast Ethernet on the optical side. This allows the Gigabit port to communicate with 100 Mbps fiber without requiring the host port to renegotiate its line speed .Key FeaturesHost Interface: SGMII (Serial Gigabit Media-Independent Interface) at 1.25 Gbps, compatible with standard Gigabit SFP cages.Optical Interface: 100BASE-FX over duplex LC fiber, supporting legacy 100 Mbps fiber links.Use Case: Ideal for connecting older 100 Mbps fiber infrastructure to modern switches or routers that only have Gigabit SFP ports.Hot-Pluggable: Can be inserted or removed without powering down the switch.Practical ConsiderationsEnsure the switch or router supports 100BASE-FX SFP modules; not all Gigabit SFP ports automatically support 100 Mbps operation.The module is typically used in enterprise or campus networks where older fiber links exist.For multi-lane high-speed ports (like QSFP28 100G), breakout functionality is used to split lanes into lower-speed connections, but this is generally for 10G or 25G lanes, not 100 Mbps .SummaryTo use a Gigabit optical port with a 100 Mbps fiber link, you cannot simply use a standard 1G SFP. You need a dedicated 100BASE-FX SFP module such as Cisco GLC-GE-100FX, which converts the Gigabit host interface to a 100 Mbps optical signal, enabling seamless connectivity between modern Gigabit ports and legacy 100 Mbps fiber infrastructure .