3Com Switch 8800 Advanced Software V5 Configuration Guide

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GR CONFIGURATION
When configuring Graceful Restart (GR), go to these sections for information you
are interested in:
■ “Introduction to Graceful Restart” on page 793
■ “Configuring Graceful Restart” on page 797
■ “Displaying and Maintaining Graceful Restart” on page 799
■ “Graceful Restart Configuration Examples” on page 799
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Throughout this chapter, the term router in this document refers to a router in a
generic sense or a Switch 8800 running routing protocols.
Introduction to
Graceful Restart
Graceful Restart
Overview
Graceful Restart ensures the continuity of packet forwarding when a routing
protocol restarts.
The mechanism of Graceful Restart works as follows: when the routing protocol
on a Graceful Restart capable device restarts, the device can notify its neighbors to
temporarily preserve its adjacency with them and the routing information. After
the routing protocol restart is finished, the neighbors help the device to
synchronize its routing information and to restore it to the state prior to the restart
in minimal time. The routing and forwarding remain highly stable during the
restart, the packet forwarding paths remain the same, and the whole system can
forward IP packets continuously. Hence, it is called "Graceful Restart".
Basic Mechanism of
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.