PLANET XGS3-24042 - Manual (Page 393)

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Chapter 46 BGP GR
46.1 Introduction to GR
Along with network development, it requires the higher availability, so HA (High Availability) is set, namely,
how to ensure packets to be forwarded and does not affect traffic operation when router control layer can not
work normally.
Usually, when a router does not work normally, neighbor in route protocol layer will detect their relationship to
be down, and is up soon. The process is called neighborhood shock. This shock will result the router shock
that will eventually result router black hole or data passed by restarted router. Finally network availability will
decrease quickly.
In order to achieve high availability, it needs upper layer route protocol to support GR (Graceful Restart). Use
GR can ensure that packets can be processed or forwarded correctly when the control layer is failing.
GR can reduce route shock, resource expend consumption of control layer and improve network stability.
What describe in this document is GR, which can restart BGP protocol without affecting forwarding process,
and forward packets in the correct path.
Figure 46-1 Aplication environment for GR
GR needs cooperation of GR-Capable router and GR-Aware router to complete. A restarted router is call
Restarting Speaker（or GR-Restarter）, and its neighbor can be called Receiving Speaker（or GR-Helper）.
Restarting Speaker is GR-Capable router while Receiving Speaker is GR-Aware router. In this way, they can
complete GR. Suppose that router R1 and R2 establish BGP neighborhood, as shown in Figure 46-1, GR
process can be described as:
Restarting Speaker（GR-Restarter）:
1. R1 and R2 negotiate GR capability through OPEN when establish original BGP neighbor.
2. When R1 is restarted, the route is kept in the interface board and guide forwarding continuously.
3. R1 establishes TCP connection with R2 again, it sets Restart state to 1 in BGP OPEN messages to show
that this router has been restarted. At the same time, it will inform the value of restart time (it is less than
Holdtime in OPEN messages) to neighbor. Additionally, it should inform neighbor what type of GR is
supported.
4. After R1 is established connection with R2 correctly, it is able to receive and deal with the update
BGP neighborhood
R1
GR-Capable
Restarting Speaker
R2
GR-Aware
Restarting Receiver