- Turn off visibility of any layers you don't want the router using.
- Turn off via visibility if you don't want it to introduce any new vias.
- Use only plain rectangles for power/ground planes that you want the router to use. (Use the rectangle tool rather than the polygon tool!)
- Make at least one connection from any plane you want the router to use to the net you want it to connect to.
- Draw continuous lines on all routing layers to outline keep-out zones.
- Use routing styles in the netlist to have per-net routing styles.
- Set the current routing style for any nets not having a defined route style in the netlist.
- Disable any nets that you don't want the autorouter to route – double-click them in the netlist window to add/remove the “*”.
- Create a fresh rat's nest. (press the O key)
- Select Show autorouter trials in the Settings menu if you want to watch what's happening.
- Choose Auto-route all rats in the Connects menu.
Note on disabled nets: If you will be manually routing these later not using planes, it is usually better to let the autorouter route them then rip them up yourself afterwards. If you plan to use a ground/power plane manually, consider making it from one or more pure rectangles and letting the autorouter have a go at it.
If you really want to muck with the router because you have a special design, e.g. all through-hole components you can mess with layer directional costs by editing the autoroute.c source file and changing the directional costs in lines 929-940 and try again. Even more mucking about with costs is possible in lines 4540-4569, but it's probably not such a good idea unless you really just want to experiment.
Just unselect the layers you don’t want (usually green and blue) by clicking on the name of the layer, then press autoroute.
Open up the netlist window. It has options for including or excluding nets from the ratlist. If you use the GTK-HID double-click a route to disable it. Make sure, only the nets you want are enabled. Optimize the rats with key O. Do Auto-route all rats.
You can have the autorouter work only within a given area by drawing a copper polygon conforming to your board’s boundary and placing it in each layer you’re trying to autoroute. You can also use this trick to autoroute only with small areas. Of course, if you accidentally have a net touching the polygon, all routes will get shorted to that net.
Connect the polygon that will become your power planes to a net and the autorouter will figure it all out. You may need some trick polygon clearances to get power routing and routing within a board outline.
This is a technological limitation of the current auto router. It is gridless and uses geometric rectangles only.
Have you tried the various clean-up tools under Connects → Optimize routed tracks ?