Smooth reflective surfaces

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.

Set the Nd high enough (around 20- or 30 for example) to make sure the object is equally reflective from all viewing angles.

Check the Force Fresnel option to make sure the reflectance of the object is only affected by the Nd and not by the Refl 0° color. More information about the Force Fresnel option can be found in the Index of Refraction - ND and K page in the documentation section.

The basic structure of an smooth reflective material like a mirror. You can download this material example here. 

Find more mirror examples in the MXM Gallery.