remebered...that Eagle would allow you to
just switch off all but one layer.
Adam Aglionby
That's not how it's done.
That's just for viewing (or printing to PnP).
You were correct the 1st time when you talked about design rules.
As Auton said it's about the "cost" of a layer
which YOU specify in the design rules.
The Autorouter uses this value to give results like what you desire.
If you set the cost of the top layer extremely high,
there will be few runs routed there
and you can go back in and edit the board
to put jumper wires where it wants to put traces.
IMO EAGLE's autorouter is pretty bad.
I advise routing by hand those runs which look hard to connect
--especially power buses--
and designing in jumper wires (top side) as necessary.
After that, turn the autorouter loose on it.