Mii-tool

eth0: negotiated 100baseTx-FD flow-control, link ok
 * 1) mii-tool

If you want to force changes (assuing your network interface can handle the changes):

-F, --force=media          force specified media technology media: 100baseT4, 100baseTx-FD, 100baseTx-HD, 10baseT-FD, 10baseT-HD, (to advertise both HD and FD) 100baseTx, 10baseT

See the Manual for more information.