You see various scenes passing on a cinema screen: fire seems to burn buildings to ashes; water seems to wreck ships; but the screen on which the pictures are projected remains unburnt and dry. Why? Because the pictures are unreal and the screen real.
Similarly, reflections pass through a mirror but it is not affected at all by their number or quality.
In the same way, the world is a phenomenon upon the substratum of the single Reality which is not affected by it in any way. Reality is only One.
Talk of illusion is due only to the point of view. Change your viewpoint to that of Knowledge and you will perceive the Universe to be only Brahman. Being now immersed in the world, you see it as a real world; get beyond it and it will disappear and Reality alone will remain.