Why is a device only linking at 100Mbps on a Gigabit switch?

A Gigabit port will negotiate down when the connected device, cable or intervening cabling cannot establish a 1000Mbps link. Check that the device itself supports Gigabit Ethernet, leave speed and duplex on automatic negotiation unless there is a specific reason not to, and test with a known-good Cat5e or better cable.

If a structured-cabling run is involved, test or bypass that run as well. A Gigabit switch cannot force a 1Gbps link through an endpoint or cable path that only supports 100Mbps.