Which feature is required for graceful restart to recover from a processor failure?

https://archive.nanog. org/meetings/nanog42/presentations/Weissner_SSO.pdf

The Stateful Switchover (SSO) feature works with Nonstop Forwarding (NSF) in Cisco software to minimize the amount of time a network is unavailable to its users following a switchover. The primary objective of SSO is to improve the availability of networks constructed with Cisco routers.