If I guess right, our navy, and Japanese navy as well, are testing the hell out of Aegis for defeating this threat.
At least, I hope to hell they are.
The US Navy has been developing anti-air and anti-missile missile systems to defend their ships since there have been ASMs. (look up the RIM-66 standard missile line) The latest variants include the SM-II blk 4 and SM-6.
Now, the DF-21D's warheads may be coming in faster than previous ASMs. They are still going to be limited by the Gs they can pull and how much aerodynamic heating they can withstand. Even so, they are relatively slow compared to say a short or medium range ballistic missile during much of it's flight. The Missile Defense Agency and the Navy have been developing the SM-3 to intercept relatively high speed ballistic missiles. During re-entry phase they are actually much slower. The point is, the tracking and guidance computers and algorithms are well known - even tested in flight already - hitting things going much faster than an RV deep in the atmosphere.
So the US Navy has the tools, in the form of the various flavors of SPY and AMDR radars, Aegis weapons system, and a couple of different variants of SM missiles. I'm sure they've already wargamed this out in simulations hundreds of times. As others have noted there have been a number of tests at various Navy "shooting ranges" off Hawaii etc. According to Google, Raytheon makes the SM line of missiles. Hmm, I might have to look into buying some Raytheon stock, I think they may be busy for quite a while with this growing threat...