A completely reflective surface can be created using a single BSDF layer. Set the Refl 90° of your material to pure white. Adjust the roughness parameter to a very low value like 0. You can increase the value (to for example 20) if you would like to create a glossier mirror.
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The basic structure of an smooth reflective material like a mirror. You can download this material example here.
mirror by beREAL |
smoke n-mirrors by abeezley |
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Find more mirror examples at in the MXM Gallery.