I have a 1 1/4" P-Trap (sink) which connects to a 2" drain pipe in the wall. Upon replacing the P-Trap I realized the 2" steel drain pipe that connects to another waste pipe at floor level is completely rusted out, broken off and separated from the sub floor pipe. In addition to that it's also rusted, broken and separated from the piece (Not sure what it's called) which connects this 2" drain pipe, the P-Trap and the other 2" pipe which runs upward for the vent. My question is this: What was the drain pipe connected to the sub floor pipe connected with? Is the pipe below all one piece with threads at the top so this smaller piece can screw into or is it more likely there is another connector type piece which is also completely rusted out that needs to be removed. Can I buy a fernco reducer and slap it over the sub floor drain pipe then connect another 2" pvc pipe to it at floor level? Or am I asking for trouble. Maybe I can buy a 1.5" PVC and slide it down into the sub floor pipe a few feet to bypass any other parts that may need to be replaced. No idea any help would be appreciated.

Best thing you could do is show us some pictures.