To render global illumination
Select Windows > Rendering Editors > Render Settings or click the Render Settings window icon.
Global illumination (in the Rendering Features section of the Features tab) is automatically enabled.
If the default settings are not sufficient to achieve the look you want (or to reduce render speed), tweak the global illumination settings until you achieve the look you want. See Caustics and Global Illumination for more information.
If your scene contains caustics, global illumination or final gather, and you would like to see those effect during mental ray IPR, in Render View select IPR > IPR > Render Settings.