Core issue
The Cisco GBIC is a hot-swappable input/output device that plugs into a Gigabit Ethernet port or slot, linking the port with the network. Cisco GBICs can be interchanged on a wide variety of Cisco products and can be intermixed in combinations of 1000BASE-T, 1000BASE-SX, 1000BASE-LX/LH, 1000BASE-ZX, or 1000BASE-CWDM interfaces on a port-by-port basis.
The Cisco SFP Gigabit Interface Converter is a hot-swappable input/output device that plugs into a Gigabit Ethernet port or slot. SFPs can be used and interchanged on a wide variety of Cisco products and can be intermixed in combinations of 1000BASE-SX, 1000BASE-LX/LH, 1000BASE-ZX, or 1000BASE-BX10-D/U on a port-by-port basis.
Resolution
You cannot configure speed or duplex mode on GBIC ports. But for certain types of GBICs (for example, 1000BASE-SX, -LX, or -ZX GBICs), you can configure the speed to not negotiate (nonegotiate) if they are connected to a device that does not support auto-negotiation.
The GBIC-module ports support only 1000 Mbps. The speed values of 10 Mbps and 100 Mbps are not supported. You cannot hardcode 100 MB on a 1000Base GBIC and it will always be 1000 MB full duplex whereas the 100BASE-FX will be 100 MB full duplex.
For more information, refer to Cisco Gigabit Interface Converter.
On the other hand, SFP modules are available in the two speeds of 100 Mbps and 1000 Mbps. You can connect a 100BASE-FX SFP FastEthernet interface converter on Gigabit SFP ports in Catalyst 3750, 3560, and 2970 Series Switches.
For more information, refer to Cisco SFP Optics For Gigabit Ethernet Applications and Cisco 100BASE-X SFP Module for FastEthernet SFP Ports.
