Hi Jim.
The DF-21 (CSS-5) is an MRBM/ICBM so there are a few different Russian equivalents. There are even more equivalents to consider when one takes into account the development of the DF-21- the D for instance, is an Anti-Shipping Ballistic Missile and another variant has been developed as an Anti-Satellite weapon- thats just two- there are several more variants and the DF-21 and its variants remain a type of missile that the PLA is keen on developing further- there could be several more roles for it in future years.
In terms of Russian 'equivalents' there are a few ICBM's in the Russian arsenal including the SS-25 (RS12M) and the road mobile Topol M (SS-27). The original DF-21 was first developed back in the late 1960s but it took quite a while to enter service (the first test only happend in 1985)- so historically an equivalent Soviet MRBM might be something like the R-12 (SS-4 Sandal) or the RT-21M (SS-20 Saber).
I hope this helps a bit

!