My 2011 MacBook Pro won't connect to ethernet at 100Mb, only 10Mb

I just upgraded my DSL from 10Mb to 100Mb. My MacBook Pro will only connect at 10Mb, even though I have tried to change the ethernet configuration. When I set the config for 100Mb, the laptop doesn't see a cable connected. Using the automatic config always reverts to 10Mb. Why?