An in-service software upgrade (ISSU) enables service providers to migrate from one software version to another. This paper describes the selection of an ISSU design, which requires a comprehensive approach that considers various permutations of redundant and non-redundant router architectures. The paper also describes Juniper's unified ISSU design, which enables service providers to smoothly upgrade the entire operating system from one major release to another. For each JUNOS release, the full integrity of the entire code and quality regression testing are achieved.