3Com Switch 4800G 24-Port - Gr Overview; Introduction to Graceful Restart; Basic Concepts in Graceful Restart

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GR OVERVIEW
Go to these sections for information you are interested in:
■ “Introduction to Graceful Restart” on page 247
■ “Basic Concepts in Graceful Restart” on page 247
■ “Graceful Restart Communication Procedure” on page 248
■ “Graceful Restart Mechanism for Several Commonly Used Protocols” on page
250
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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.
■ 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.
■ GR Helper: The neighbor of the GR Restarter, which helps the GR Restarter to
retain the routing information. It must be Graceful Restart capable.
■ 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.
■ 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.