Graceful Restart and Non Stop Routing both allow for the forwarding of data packets to continue along known routes while the routing protocol information is being restored (in the case of Graceful Restart) or refreshed (in the case of Non Stop Routing) following a processor switchover. When Graceful Restart is used, peer networking devices are informed, via protocol extensions prior to the event, of the SSO capable routers ability to perform graceful restart. The peer device must have the ability to understand this messaging. When a switchover occurs, the peer will continue to forward to the switching over router as instructed by the GR process for each particular protocol, even though in most cases the peering relationship needs to be rebuilt.
Essentially, the peer router will give the switching over router a "grace" period to re-establish the neighbor relationship, whilecontinuing to forward to the routes from that peer.
http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/high-availability/solution_overview_c22-
487228.html

Section: Data Center Compute

Explanation/Reference: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/system_management/ configuration/guide/sm_nx_os_cg/sm_9snmp.pdf