At Anode - always oxidation take place.
∵ Reduction potential of E° (Al^+3/Al) = -1.66 V
it means, oxidation potential of E° (Al/Al^+3) = 1.66 V
and similarly oxidation potential of E°(Zn/Zn^+2) = 0.764 V
∵ oxidation potential of Al is greater than the oxidation potential of Zn
because of that Al act as an Anode and Zn as a Cathode.