This article is really more for the folks at Autodesk than anyone else because they are the only ones who can fix the problem I am going to discuss in this article.

When you design large projects in AutoCAD or AutoCAD Architecture you typically rely on Xref's to assemble a composite file or model.  When it comes to AutoCAD Architecture, specifically, there is even a Project Navigator tool that assists in creating composite model files. 

When you create Renderings you typically want to use a composite model file because of numerous factors concerning the Rendering process; among which are settings you don't want in your production files (sun), props and the possibility of crashes (often occur in rendering files).  With architectural projects, you may also want different composite files for things like interior and exterior Renderings.  So, what's the problem?

The problem is that Render Materials don't abide by the rules of Xref's.  Once a model file has been Xref'd into a Rendering base file, the Render Materials become local.  Even if you Detach the source file(s) and Attach them again, the original Render Materials will remain.  For someone trying to manage multiple Render files this is a nightmare.

In the illustration to the right I show how a Render material change in the source file does not match the Render material settings in the Render file configured by Xref's.

In addition to the Render materials not abiding by the rules of Xref's, transferring settings between files is also a nightmare.  If you attempt to Drag-n-Drop (there's no Copy-n-Paste option) a Render Material of the same name from one source into another you get a duplicate material name with something like a suffix of "(1)" - there's no option to override existing material.

Read Using the Tool Palette to Import and Override an Existing Material in my Review on Rendering in AutoCAD 2011 and AutoCAD Architecture 2011 for a solution to exporting and importing Render material settings.

Note:
Another solution I found to deal with this problem is to Delete all of the Render materials you want to update in the Render file.  As long as they are not materials directly attached to objects within the Render file, you won't lose the connection because that is established in the source model file.  When you Reload the Xref(s), the Render materials, with their changed settings, will import correctly.

It would be great to have a setting where Render materials respect Xref's.