I'm replying my own post in desesperation. No luck up to now and
nobody has given me a clue on how to set up the "metric" parameter of
an interface. This is soooo easily done in windows, I'm sure it can
be done with linux too.

The man page of ifconfig says that you may set the metric, but I
haven't succeded in making it work. The command "ifconfig eth0 up
metric 10" return the following error: "SIOCSIFMETRIC: Operation not
supported". The interface does always get metric=1 as a glued and
unmovable default...

Is my question clear ? Has Anybody any clue?