Graceful Restart ensures the continuity of packet forwarding when a routing protocol restarts.

The mechanism of Graceful Restart works as follows: after the routing protocol on a Graceful Restart capable device has restarted, the device will notify its neighbors to temporarily preserve its adjacency with them and the routing information. The neighbors will help the restarting device to update its routing information and to restore it to the state prior to the restart in minimal time. The routing and forwarding remain highly stable across the restart, the packet forwarding path remains the same, and the whole system can forward IP packets continuously. Hence, it is called “Graceful Restart”.

Basic Concepts in Graceful Restart

A router with the Graceful Restart feature enabled is called a Graceful Restart capable router. It can perform a Graceful Restart when its routing protocol restarts. Routers that are not Graceful Restart capable will follow the normal restart procedures after a routing protocol restart.

l           GR Restarter: Graceful restarting router, the router whose routing protocol has restarted due to administrator instructions or network failure. It must be Graceful Restart capable.

l           GR Helper: The neighbor of the GR Restarter, which helps the GR Restarter to retain the routing information. It must be Graceful Restart capable.

l           GR Session: A Graceful Restart session, which is the negotiation between the GR Restarter and the GR Helper. A GR session includes restart notification and communications across restart. Through this session, GR Restarter and GR Helper can know the GR capability of each other.

l           GR Time: The time taken for the GR Restarter and the GR Helper to establish a session between them. Upon detection of the down state of a neighbor, the GR Helper will preserve the topology and routing information sent from the GR Restarter for a period as specified by the GR Time.

Graceful Restart Communication Procedure

Configure a device as GR Restarter in a network. This device and its GR Helper must support GR or be GR capable. Thus, when GR Restarter restarts, its GR Helper can know its restart process.

The communication procedure between the GR Restarter and the GR Helper works as follows:

1)         A GR session is established between the GR Restarter and the GR Helper.

Figure 1 A GR session is established between the GR Restarter and the GR Helper

As illustrated in Figure 1, Router A works as GR Restarter, Router B, Router C and Router D are the GR Helpers of Router A. A GR session is established between the GR Restarter and the GR Helper.

2)         GR Restarter restarting

Figure 2 Restarting process for the GR Restarter

As illustrated in Figure 2. The GR Helper detects that the GR Restarter has restarted its routing protocol and assumes that it will recover within the GR Time. Before the GR Time expires, the GR Helper will neither terminate the session with the GR Restarter nor delete the topology or routing information of the latter.

3)         GR Restarter signaling to GR Helper

Figure 3 The GR Restarter signals to the GR Helper(s) after restart

As illustrated in Figure 3, after the GR Restarter has recovered, it will signal to all its neighbors and will reestablish GR Session.

4)         The GR Restarter obtaining topology and routing information from the GR Helper

Figure 4 The GR Restarter obtains topology and routing information from the GR Helper

As illustrated in Figure 4, the GR Restarter obtains the necessary topology and routing information from all its neighbors through the GR sessions between them and calculates its own routing table based on this information.

Graceful Restart Mechanism for Several Commonly Used Protocols

Comware supports Graceful Restart based on Boarder Gateway Protocol (BGP), Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), and Label Distribution Protocol (LDP).