In a modular system there are two supervisors which ensures redundancy in event of Hardware and software failures. At any given time, only one supervisor is in control (managing most hardware, including all the linecards). We call it the active supervisor. The other supervisor is called standby supervisor, which serves as a backup in case the active supervisor fails. Stateful switchover is the transition when the standby supervisor takes over control of the entire system from the active supervisor (and therefore becomes the new active). This document describes PIM SSO works and its limitations.