There are various posts in the ISRO and every job post has different eligibility criteria.You can check  complete details about various jobs in ISRO by below link:

Https://www.isro.gov.in/careers

To recruit the Scientist/Engineer ‘SC ,ISRO conducts written test and Student should have BE/B.Tech or equivalent qualification in first class with an aggregate minimum of 65%  marks  or  CGPA  6.84/10  (average  of  all  semesters  for  which  results  are  available). Candidates applying with qualification of AMIE/Grad IETE should have 65% marks or CGPA 6.84 in Section ‘B’ alone.There is direct Job opportunity for IIST B.tech graduate student also in ISRO. The student should have completed the B.Tech. Programme in four years or the dual degree programme in five years, with a minimum Cumulative Grade Point Average(CGPA) of 7.5 out of 10.

You can check complete details about ISRO absorption of IIST students by below link:https://www.iist.ac.in/admissions/undergraduate