1-11 GR OverviewGo to these sections for information you are interested in:z Introduction to Graceful Restartz Basic Concepts in Graceful Restartz Graceful Restart Communication Procedurez Graceful Restart Mechanism for Several Commonly Used ProtocolsThroughout this chapter, the term router refers to a router in a generic sense or a Layer 3 switch runningrouting protocols or MPLS protocol.Introduction to Graceful RestartGraceful Restart ensures the continuity of packet forwarding when a protocol restarts.The mechanism of Graceful Restart works as follows: When the protocol on a device restarts, thedevice will notify its neighbors to temporarily preserve the routing information and adjacencyrelationship with the device. After the protocol restarts, the neighbors will help the restarting device toupdate information (including various topology, routing and session information maintained byrouting/MPLS related protocols that support Graceful Restart) and to restore it to the state prior to therestart in minimal time. No route flapping occurs during the restart, the packet forwarding path remainsthe same, and the whole system can forward data continuously. Hence, it is called “Graceful Restart”.Basic Concepts in Graceful RestartA router with the Graceful Restart function enabled is called a Graceful Restart-capable router. It canperform a Graceful Restart when its routing protocol restarts, ensuring consistent forwarding services.Routers that are not Graceful Restart capable will follow the normal restart procedures after a routingprotocol restart.z GR Restarter: Graceful restarting router, the router whose routing protocol has restarted due toadministrator instructions or network failure. It must be Graceful Restart capable.z GR Helper: The neighbor of the GR Restarter. It helps the GR Restarter to retain the routinginformation. It must be Graceful Restart capable.z GR Session: A Graceful Restart session, which is the negotiation between the GR Restarter andthe GR Helper. A GR session includes restart notification and communications during restart.Through this session, GR Restarter and GR Helper can know the GR capability of each other.
