Is it obligatory to wash the body parts that one suspects water does not reach during ghusl, or is it merely recommended?

If a person performing ghusl (definition) doubts having washed a limb during the ghusl, he must wash it, unless it is a result of obsessive whisperings (waswasa), in which case he should disregard it. However, if the doubt occurs after completing the ghusl, he should not pay attention to it. It is obligatory to ensure water reaches all folds of the body, inside the navel, the inner parts of the ears, the armpits, between the buttocks, between the toes, and the red part of the lips.