GLC-GE-100FX: Cisco's 100BASE-FX SFP for Gigabit Ethernet Ports
GLC-GE-100FX is a Cisco SFP module that lets a Gigabit Ethernet port on a Cisco switch or router carry a 100BASE-FX optical link. A standard 1000BASE-SX or 1000BASE-LX SFP cannot simply be configured to run at 100 Mbps because its optical PHY is fixed at 1 Gbps. GLC-GE-100FX exists specifically to fill that gap: it presents a 1G SGMII signal to the host port while running 100 Mbps Fast Ethernet on the optical side.
The module is the only practical way to attach a legacy 100BASE-FX fibre run to a modern Cisco switch that only exposes Gigabit Ethernet SFP cages. It is also the option to reach for when a remote site terminates 100 Mbps fibre but the local chassis has been upgraded to a GE-only SFP edge.
How GLC-GE-100FX runs Fast Ethernet over a Gigabit port
The host side of the module presents a SGMII (Serial Gigabit Media-Independent Interface) signal at the standard 1.25 Gbps SerDes rate, which is what the GE SFP cage expects to see. SGMII can carry 10, 100, or 1000 Mbps payloads over the same 1.25 Gbps physical lane by repeating each byte at the slower rates, so the host MAC and the SFP stay rate-aligned without the host port renegotiating its line speed.
Inside the module, the SGMII signal is converted to a 100BASE-FX optical signal that drives the LC duplex fibre pair at 100 Mbps. From the operator's perspective the host port reports a 100 Mbps full-duplex link and the optical span carries standard 100BASE-FX, even though the SFP cage itself is wired for Gigabit.
Optical specifications
The optical interface follows IEEE 802.3u (100BASE-FX): 1310 nm nominal wavelength, multimode fibre, LC duplex connector, and a maximum reach of 2 km on 50/125 or 62.5/125 micron MMF. The module supports Digital Optical Monitoring (DDM/DOM) for live link telemetry. Specifications above are cross-referenced from secondary vendor sources; for authoritative values consult Cisco's official product documentation.
Where this module is supported
Platform support for the GLC-GE-100FX varies by chassis family, line card, and network module. The authoritative source is Cisco's 100-Megabit Ethernet SFP Modules Compatibility Matrix, which lists each supported chassis, the line cards or network modules required, and the minimum software train. Confirm support for the specific platform and software version in production before ordering.
On dual-purpose uplink ports that auto-select between copper and fibre media, the SFP cage may need an explicit media-type configuration before it activates the optical PHY.
Configuration notes
Once inserted, the module presents a fixed 100 Mbps full-duplex link to the host port via SGMII, so explicit speed configuration is generally not required on the host side. Two operational details are worth confirming during commissioning: the port should be configured to the correct media type if the chassis supports both copper and fibre on the same port, and duplex should be matched on the far-end device because duplex mismatches produce late collisions that look like fibre faults. The relevant CLI commands and behaviour vary by Cisco platform and software train, so consult the port configuration guide that ships with the chassis.
EdgeOptic supplies a Cisco-coded compatible for this part. Specifications, pricing, and the datasheet are on the GLC-GE-100FX product page. For sizing across multiple sites or to confirm support against a specific Cisco platform, contact our sales team.
GLC-GE-100FX FAQs
What is the difference between GLC-GE-100FX and GLC-FE-100FX?
Both modules carry a 100BASE-FX optical link, but they are designed for different host port types. GLC-GE-100FX goes into a Gigabit Ethernet (GE) SFP port and uses SGMII on the host side to bridge the GE host PHY down to a 100 Mbps optical lane. GLC-FE-100FX goes into a Fast Ethernet (FE) SFP port that already runs at 100 Mbps natively, so it does not need the SGMII bridging. The GLC-GE-100FX runs at higher power because of the SGMII conversion, which can be a consideration in fanless or compact switches. Confirm which module matches the SFP cage in your specific Cisco platform.
How does the GLC-GE-100FX let a Gigabit Ethernet port run at 100 Mbps?
The module presents a SGMII (Serial Gigabit Media-Independent Interface) signal to the host SFP cage at the standard 1.25 Gbps SerDes rate, which keeps the GE port's PHY happy. On the optical side, an internal 100BASE-FX PHY drives the LC fiber pair at 100 Mbps. SGMII carries 10/100/1000 Mbps payloads over the same 1.25 Gbps SerDes link by repeating each byte at the lower rates, so the host MAC and the optical PHY stay rate-aligned without any speed renegotiation on the host port. This is why a standard 1000BASE-SX or 1000BASE-LX SFP cannot be configured to run at 100 Mbps: the optical PHY in those modules is a fixed 1G PHY with no 100BASE-FX path.
Will the GLC-GE-100FX work in a non-Cisco SFP cage?
The SGMII host interface itself is industry-standard, so the optical layer can interoperate with non-Cisco hosts that expose a SGMII-capable SFP cage. The module's EEPROM, however, is Cisco-coded, and some non-Cisco platforms reject unrecognised vendor IDs by default or display warnings. HPE Aruba switches typically require an allow-unsupported-transceiver style command, and other vendors have similar overrides. Verify against your switch vendor's transceiver compatibility documentation before deploying.
Do I need to configure speed or duplex on the port after inserting the GLC-GE-100FX?
The module presents a fixed 100 Mbps full-duplex optical link via SGMII to the host port, so explicit speed configuration on the host side is generally not needed. Two operational notes: on dual-purpose uplink ports that auto-select between copper and fiber, the SFP cage may need an explicit media-type command set before the optical PHY activates; and duplex mismatches with the far-end device are a known failure mode that produces late collisions, so hard-coding duplex full on the neighbouring device is a defensive practice. Always check the port configuration guide for your specific Cisco platform.
Which Cisco switches and routers support the GLC-GE-100FX?
Platform support varies across Catalyst, ISR, and ASR families and depends on the line card or network module installed. The authoritative list is Cisco's 100-Megabit Ethernet SFP Modules Compatibility Matrix. The matrix lists each chassis, the supported line cards or network modules, and the minimum software train.