Well firstly it should be NIT not NIIT.

Secondly, through JEE advanced aspirant can be admitted only in IIT not in NIT. 

Well brother, I can understand your situation. I know you have worked hard for JEE advanced but I believe with 21.7k CRL rank you won't be getting decent IIT which I can recommend you.

Though with OBC, there might be an increased chance of your's getting a seat but with such a rank it is very hard to get.

As, opting a very low rank IIT core branch will bring nothing good for you which lags even the permanent faculty and infrasture to nurture you.

So, I would suggest you to prefer state engineering colleges or decent NIT's.

Also, I would recommend you to do take part in JoSAA and go to through all the round even the spot round. May be, some good fortune can turn your destiny.

hello priyanshu,

as you secured a rank of 21730 it very good

but the BEST IITs cannot achived by with this ranks

because the competition for the IITs will be at a peak range

you have a chance of getting in to the north side IITs

you can grab them easily

all the  best