H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 209)

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GR Overview
Go to these sections for information you are interested in:
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Introduction to Graceful Restart
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Basic Concepts in Graceful Restart
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Graceful Restart Communication Procedure
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Graceful Restart Mechanism for Several Commonly Used Protocols
Throughout this chapter, the term router and the router icon refers to a router in a generic sense or a
Layer 3 switch running routing protocols.
Introduction to Graceful Restart
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.
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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.
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GR Helper: The neighbor of the GR Restarter, which helps the GR Restarter to retain the routing
information. It must be Graceful Restart capable.
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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.
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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.