OSPF Graceful Restart allows a router to restart while it’s neighbors continue to forward packets to the restarting router as if it was up and running. This is referred to as routing through a failure, instead of routing around a failure which would be OSPFs usual response to a router going down.
This was introduced in IOS when Non Stop Forwarding (NSF) was introduced. NSF and GR are both configured using nsf commands.
The router restarting is said to be in restart mode while its neighbors are in helper mode during the graceful restart.
helper neighbors have to ignore the fact that there is a lack of hellos keeping the ospf neighborship up during the restart, it ignores them during a predefined grace period and considers the restarting router to be fully adjacent during this time.
Any router can act as a helper if its supported in the IOS, however only routers with specific hardware support can perform restart functions, since there is a need to have autonomous forwarding hardware in the event of a failure. To differentiate between the two types of devices cisco refers to helper devices as NSF-aware and restart devices as NSF-capable.