Theres an elbow or turn in the pipe and something ain't moving.Pipes
are probably in the wall too.
Do you have any pre-schoolers who get in there when no one is looking?
The fact that it is the upstairs bathroom makes it a sure thing it is a
solid clog.
Removing the toilet and using a snake is the only way I know without major
repairs.
If you have a shower bath,or sink in the same bathroom and they drain fine....
Your easiest repair is to pull the toilet and unclog the drain with a snake.
Edit:I see xpuser357 has been there done that. :grin:
Please forgive me in advance for the picture.
http://i94.photobucket.com/albums/l9...etshadow-2.jpg