Cisco Nonstop Forwarding (NSF) with Stateful Switchover (SSO) provides increased network service availability and protection against unplanned downtime due to hardware or software problems.
Cisco Nonstop Forwarding and Stateful Switchover are designed to be deployed together. NSF relies on SSO to ensure that links and interfaces remain up during switchover, and that lower layer protocol state is maintained. NSF is, however, configured separately and it is possible to enable SSO without NSF.
https://www.cisco.com/en/US/technologies/tk869/tk769/technologies_white_paper0900aecd801d c5e2.html