This study will evaluate the safety, tolerability, and descriptive efficacy of BIIB017 in pediatric participants with relapsing-remitting multiple sclerosis (RRMS) and to assess the pharmacokinetics (PK) of BIIB017 in pediatric participants with RRMS in Part 1. In Part 2, the study will evaluate the long-term safety of BIIB017 and further describe safety and the long-term multiple sclerosis (MS) outcomes after BIIB017 treatment in participants who completed the study treatment at Week 96 in Part 1 of the study.