Firstly you have to ensure that your CGPA is equivalent to 50% in order to be eligible to take CAT. Next step is to secure 90 percentile or above in CAT. Along with CAT percentile, IITs also consider past profile. Even if you have less CGPA, if you can balance it out with other qualities like work experience, extra curricular activities etc. you might have a chance at getting interview call by IIT Madras or Roorkee or Kanpur. IIT Delhi, Bombay and Kharagpur are tough baits. However, never lose hope and prepare hard for CAT 2017 to ensure a high percentile.