  The diffuse extragalactic gamma-ray background (EGRB) has been derived by
various groups from observations by the Energetic Gamma Ray Experiment
Telescope (EGRET) instrument on the Compton Gamma Ray Observatory (CGRO). The
derived EGRB consists of gamma rays that may come from astrophysical
components, such as from unresolved extragalactic point sources (blazars,
normal galaxies, etc.), true extragalactic diffuse emission, misattributed
diffuse signals from the Galaxy and other celestial sources, and an irreducible
instrumental background due to gamma rays produced by cosmic-ray (CR)
interactions in the EGRET instrument. Using the Gamma Ray Large Area Space
Telescope (GLAST) simulation and reconstruction software, we have investigated
the magnitude of the irreducible instrumental background in the GLAST Large
Area Telescope. We re-scale our results to the EGRET and present preliminary
results of our study and its effect on current estimates of the EGRB.
