Don't have to round them up; make things too uncomfortable for them and they will go back where they belong.
1} e-verify for all hires
2} Draconian fines for hiring illegals; 1/2% of the highest of previous 5 years GROSS income for each illegal & the actual person who did the hiring gets $5,000 fine and 6 months in jail for each illegal hired
3} e-verify for housing rentals/purchases, rent to an illegal and same 1/2% fine & 6 months jail
4} Make any form of welfare to an illegal, illegal and the person signing them up gets $5,000 fine and 6 months in jail per illegal, the illegal gets 1 year in jail and deported
Can't work, can't put a roof over their head and can't eat; they'll go back where they came from the same way they got here.