Technically, it will work on older AOS (6.1) on the 6x0, however, a lot of development and fixes have gone into the feature since then. Beginning with 6.2 it is no longer supported on the 6x0 series. It will work fine on the 3x00 series. It will also work on the new 7000 series (7005 / 7010 / 7030) and the 7200 series.