Introduction To Graceful Restart; Basic Concepts In Graceful Restart - 3Com MSR 50 Series Configuration Manual

105
Introduction to
Graceful Restart
Basic Concepts in
Graceful Restart
GR O
VERVIEW
When configuring GR, go to these sections for information you are interested in:
"Introduction to Graceful Restart" on page 1957
■
"Basic Concepts in Graceful Restart" on page 1957
■
"Graceful Restart communication procedure" on page 1958
■
"Graceful Restart Mechanism for Several Commonly Used Protocols" on page
■
1960
Graceful Restart (GR) 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".
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. The basic concepts involved in GR are
introduced as follows:
GR Restarter
GR Restarter is a restarting router, whose routing protocol has restarted due to
administrator instructions or network failure. It must be Graceful Restart capable.
GR Helper
GR Helper is the neighbor of the GR Restarter, and helps the GR Restarter to retain
the routing information. It must be Graceful Restart capable.
GR Session
GR session 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.