Questions have come up on exactly what is the criteria under the Voting Rights Act that deals with redistricting.
The Department of Justice summarizes Section 5 of the VRA here.
This is an AP article regarding North Carolina is currently challenging certain provisions and the case is now at the Supreme Court.
Also here is the Washington Post story on DOJ officials who thought the Texas plan was illegal.
And for those who want more the ACLU has a redistricting guide with a lot of FAQs on redistricting and the VRA.