7520 Series and 7720 Series switches can be stacked with themselves. They can also be stacked with each other using Native V400 stacking (SummitStack-V400), or Native-V400-Alternative-configuration (using the long range AOC optics). Both the 7520 Series and the 7720 Series have the same capability in terms of feature scalability and the speed, so either of them can be the primary switch or the backup switch.