Immigration laws have a massive effect on national security; thus, great deference is given to the president's actions in this realm.
Any law that would inhibit the flexibility of the Presidency to respond to changing world conditions should be adopted only with the greatest caution; thus, the rational basis test is used.
If an immigration restriction imposed by the President has a conceivable, permissible purpose, then the restriction is constitutional.
From here: Trump issued an Executive Order that suspended immigration from countries that did no provide adequate information for informed entry determinations. All of these countries were also identified as posing heightened terrorism risks.
Although Trump was previously on record expressing his interest of implementing a "Muslim ban," this EO was constitutional, as it passed the scrutiny of the rational basis test.