1000BASE-LX and 100BASE-LX are very different standards. 1000BASE-LX SFP (GLC-LH-SMD) and 100BASE-LX SFP (GLC-FE-100LX) are two module types, one for GE and one for FE slots. The GLC-LH-SMD does not operate at 100Mbps. You will need to use a 100Mb module to connect to another device at 100Mb than you visit SwitchTechSupply. Check optical transceiver compatibility with your equipment by reviewing the Cisco Fast Ethernet SFP module compatibility document and choose the one it says is compatible.