The program stores the folder path of the referenced drawing. Each time you open or
plot the drawing, or use the Reload option in the External References palette to update
the xref, the program checks the folder path to determine the name and location of
the referenced drawing file.

IIf the name or location of the drawing file has changed, the program cannot locate
or reload the xref, and it displays an error message that displays the folder path
and name of the missing drawing file.

In the drawing, at each insertion of the missing xref, the program displays text that
displays the folder path of the missing xref. You can use the XREF Path option to
update or correct the path.

Along with error messages being displayed at the Command prompt, a task dialog box
might be displayed that allows you to ignore all missing xrefs or update their folder
locations. You can use the External References palette to update the locations of
the unresolved references.

To avoid these errors make sure that when you transfer or distribute drawing files
that have xrefs attached, you also include all the referenced files.

Changing Nested Xref Paths

When a drawing is opened and a nested xref is loaded, the program attempts to find
the xref in the original xref path first. If the xref is not found, the following
search is initiated in the order shown:

• Current folder of the host drawing
• Project search paths defined on the Files tab in the Options dialog box and in the
  PROJECTNAME system variable (not available in AutoCAD LT)
• Support search paths defined on the Files tab in the Options dialog box
• Start In folder specified in the Microsoft ^آ® Windows ^آ® application shortcut

This search order helps ensure that revisions made to the xref are reflected in the
current drawing, and also makes it possible for the xref to be found if its folder
path has changed.

Resolving Circular References

A drawing that contains a sequence of nested references that refers back to itself
is considered a circular reference. For example, if drawing A attaches drawing B,
which attaches drawing C, which attaches drawing A, the reference sequence ABCA
is a circular reference.

If the program detects a circular reference while attaching an xref, a warning is
displayed asking you if you want to continue. If you respond with yes, the program
reads in the xref and any nested xrefs to the point where it detects the circularity.
If you respond with no, the process is halted and the xref is not attached.

If a circular reference is encountered while loading a drawing, an error message is
displayed and the circular reference for the current session is broken. For example,
if you have the circular reference ABCA, and you open a.dwg, the program detects and breaks the circularity between c.dwg and a.dwg. The following error message is displayed:

Breaking circular reference from C to current drawing.