Click
a DGN underlay and right-click in the drawing area. On the shortcut menu,
click DGN Clip.
At
the command prompt, enter n (New Boundary).
Enter p (Polygonal)
or r (Rectangular), and then draw
the boundary on the underlay.
If you are drawing a polygonal boundary, you
are prompted to specify consecutive vertices. To finish drawing
a polygon, press ENTER or right-click anywhere in the drawing area.