I recently worked on an interesting problem with regards to getting some DXF/DWG 3D surfaces and 3D lines from a software package into Inventor. The challenge was to get the model into Inventor and then have it update in the originating software, which would then update Inventor and any other parts that have been modelled off the import.
This sounded like a breeze because AutoCAD works with AnyCAD and when you update it will update. Easy peasy lemon squeezy! Well not really.
One of the issues that we found was that AutoCAD AnyCAD works best for floor plans (2D). The idea is that you have a 2D which you then link as a base view to create your 3D parts not really 3D.
For 3D parts to be linked and updated you would need STEP files. Now how do you generate this because if you have vanilla AutoCAD there is no option to crate a STEP file. If you have the Product Design and Manufacturing Collections or the AutoCAD with specialised toolsets then you have access to AutoCAD Mechanical.
With AutoCAD Mechanical you use the STEPOUT command to crate the STEP file. Please note that this does significantly increase the size of the model. You are then able to AnyCAD that model back into Inventor and be able to reference it in your model and if the original STEP model is changed then it will update in Inventor and whatever you have modelled off the reference will also update.
Please note that when working with many surfaces, 3D lines and points it does take a while to AnyCAD that object in, so be patient and make sure that you have adequate hardware to work with these files.