Question : MAcbook Pro refuses to connect via ethernet but works via wifi

My mbpro with OS X Snow Leopard including all latest updates refuses to connect to my draytek router vigor 2910.
Connect via Airport works fine, but when connection the ethernet cable, the interface en0 has a self assigned IP.

The whole scenario worked without a problem the last time (about four weeks ago), no updates were made to the mbp since then (... that I know of. I have auto updates enabled:-).

Intermittedly, I was able to get it running with "DHCP with manual IP", but the connection is sloppy at best.

Additional informartion:
An Airport Extreme router is connected to the Draytek router in bridged mode. the mbp connects succesfully to it's 802.11n wifi and to the internet.
I tested with the cable going to the Aiprot Extreme and directly to the Draytek, both scenarios show the same symptoms.

Answer : MAcbook Pro refuses to connect via ethernet but works via wifi

Try resetting the PRAM by cold booting while holding down the command-option-p-r key combination until you hear the startup chime a third time.

If that doesn't solve your problem, download and install the OS 10.6.3 combo updater (even if you are already at 10.6.3):

http://support.apple.com/kb/dl1017