The aluminium hull of the BMD-2 is carried over from the BMD-1, and the turret is made of steel, just like with its predecessor. The vehicle is very light, but that is not to say that the vehicle has distinguishably poor protection per se; Although the BMD-2 is much lighter than most other IFVs, it is also much, much smaller than most other IFVs, which means that it retains armour density roughly equal to that of a volumetrically larger and correspondingly heavier vehicle.
The hull is made of aluminium alloy, while the turret is made of steel. The frontal arc can withstand .50 caliber machine gun fire at reasonable distances, and the sides can resist 7.62mm machine gun fire with good guarantees - slightly worse than the non-airborne BMP-2 nominally, but superior to the M113, a similarly aluminium-cladded armour personnel carrier.
The aluminium used for the BMD-2 is alloy ABT-101, same as with the BMD-1. According to several research papers written on the subject, the effectiveness of aluminium armour may reach up to 50% of steel, but non-armour grade aluminium alloys are typically only around 40% as effective (or less). An example of this would be 5083 alloy, which were only 34% as effective as steel for the same thickness, given that the plates used were above a certain thickness threshold. ABT-101 was specially developed for use as armour, and because of that, it had more suitable properties, giving it significantly better performance (up to 45% as effective as steel armour). However, because of aluminium's generally worse hardness, it is much less capable of deflecting ballistic threats than typical armour-grade steel for the same thickness, so aluminium armour does not gain as much protection from angling as hard, armour-grade steel would. ABT-101 has a hardness of approximately 145 BHN, harder than mild steel and harder than 7039 aluminium alloy, which is known to be used in American designs like the M551 Sheridan and M2 Bradley, and much, much harder than the 5083 alloy used in the M113, which had a hardness of just 75 BHN, but much lower than typical RHA steel, which typically ranges from 220 BHN to 300 BHN in hardness. The comparatively greater hardness affords the BMD-2 better performance against bullets of all types compared to foreign aluminium armour of the same weight, and certainly significantly greater potential as sloped armour.