After configuring a networkconnection (wire) using ceni/static/built-in the connection dies within few seconds.
It seems networkmanager (NM) is started by default (no idea how to stop it) and controls all network activities.
Using NM to configure a connection works fine although I must say that on a live CD it will displease each new user by asking for a p/w *after* the settings are typed in. One then will notice there is no p/w set .... and so on ... you will curse this behaviour.
To me it seems the best to disable (not autostart) NM or remove ceni.