What is the primary requirement for deploying OSPF graceful restart on a router?

Aggressive hellos and keepalives are unrelated to graceful restart and actually work against it by detecting the failure quickly and tearing down the adjacency before recovery completes.

OSPF graceful restart (RFC 3623) works by having the restarting router send Grace-LSAs to notify neighbors that it is restarting. Adjacent routers must be configured in helper mode, which instructs them to maintain the adjacency and continue forwarding traffic as if the neighbor were still present for the duration of the restart window.

Fast Layer 2 down detection would cause neighbors to immediately tear down the OSPF adjacency upon link failure, defeating the purpose of graceful restart entirely.

While adjacent routers do forward traffic during the failure as part of helper mode behavior, the specific technical requirement is that they be explicitly enabled with graceful restart helper mode, not just that they generically forward traffic.