No discussion of system high availability is complete without discussing in-service software upgrade (ISSU). One of the biggest challenges faced today in the WAN is availability in the face of vulnerabilities, bugs, outages, or the introduction of new services. All these events require upgrading at least a subset of system software (and at most the entire system software) without impacting the services.
The Cisco ASR 1000 supports IOS ISSU across all the systems, while maintaining the backward compatibility with the older uniprocessor systems booting off of IOS as one monolithic binary file. This chapter covers the need for ISSU and ISSU procedures for the ASR 1000.