You can attach DGN files as underlays to a drawing file.
DGN files are referenced and placed in drawing files similar to raster image files, but they are not actually part of the drawing file. The DGN underlay is linked to the drawing file through a path name. Linked DGN paths can be changed or removed at any time. By attaching DGN underlays using linked DGN paths, you can place DGN underlays in your drawing without greatly increasing the drawing file size.
You also have the option to drag DGN underlays directly into the drawing. Once you drop the DGN file, you are prompted for an insertion point, scale, and rotation.
Once you've attached a DGN underlay, you can reattach it multiple times, treating it as if it were a block. Each underlay has its own clip boundary and its own settings for contrast, fade, and monochrome. However, you cannot bind the underlay to the drawing and you cannot edit or modify the underlayâ€™s drawing content.
When a DGN file is attached as an underlay, any level structure it may have is flattened into a single layer. The underlay is placed on the current drawing layer.
If you work with drawings that have DWG references (xrefs), there is a chance that one of the DWG references may have a DGN underlay attached. In this situation, the DGN geometry will be visible in the parent drawing.
For example, Drawing A includes a DGN underlay showing some mechanical details. You need the content of Drawing A attached to your current drawing, Drawing B. If you attach Drawing A as an external reference to Drawing B, the DGN underlay that was already attached to Drawing A will also be present.
All of the property settings made to the underlay in the DWG reference, such as clipping boundaries, will appear as they do in the parent drawing.
If you attempt to attach a DGN file that contains 3D objects, you will receive a warning message.