I had this problem also.
Config: 2 Dell R610 servers each with 4 x 1G ports.
The problem was ONE port on ONE server was reporting 100Mbps... I unplugged, replugged. I swapped cables. Nothing changed. Only when I changed ports on the switch did it make a difference. I am still not sure why. I need to test another 1Gbps device on that port.
