How do you combine volumes/polygons from steps into one ramp? If I had a volume for each step, I'd have to do A* through each step.
Also, how do you do A* through the volumes/polygons? Do you have entrances and exits between each volume? How would you cut a shortcut across several volumes if there's a direct path?
This link has a full explanation of how Navmeshes are created and used:
http://www.critterai.org/nmgen_study
Edit:
Read your question wrong, sorry. The link should help you.
Simple answer - Nav meshes are usually a seperate mesh simplification of the actual scenery geometries.
The stairs would often be simplified into a flat ramp surface
I'm sure you can find something to read rather than having us copy and paste it here. There's no shortage of info on navmeshes on the web.
