PLANET XGS3-24042 - Manual (Page 398)

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Chapter 47 OSPF GR
47.1 Introduction to OSPF GR
OSPF Graceful-Restart（short for OSPF GR） ，is used to maintain data forwarding correctly and flow of crucial
service is not interrupted when routing protocol restarts or switchover of layer 3 switches between active
master and standby master. It is one of high availability technologies.
So far, the high layer 3 switches usually adopt a design for separating control and forwarding. The control
module for counting routing protocol at master control board, but data forwarding module is at liner card. As a
result, it will not affect data forwarding on line card when the master control board is restarted. So the device
supporting GR is generally a chassis device and has two master control boards.
Since standard OSPF protocol (RFC2328) does not support GR, it will lead to flow cut off and routing surge
when routing protocol is restarted or switchover between active master and standby master for various
reasons. For example, as shown in below figure, when S1 occurs switchover, the neighborhood relation
between S1 and S2 will lose, at that time S2 will send Router-LSA to S3 and S4 and this LSA does not include
the link between S1 and S2. After S3 and S4 received LSA, they will count routing protocol again. The result
will not include the link between S1 and S2. After S1 finishes the switchover, it will establish neighborhood
relation with S2 and synchronize database, this action leads S2, S3 and S4 to count routing again. However,
switchover of S1 will result routing shiver, which is not accepted by some networks with high requirement for
performance.
S1
S2
S3
S4
Figure 47-1 typical application scene
OSPF GR described in RF C3623 is come up for the above state. Its basic idea is that if the network topology
keeps stabilization during the switchover and layer 3 switch can maintain the same forwarding list, then its
neighbor can maintain their relationship, which can make the switch on its forwarding path still. If S1 and S2
support and enable GR, the liner card of S1 will keep the traffic forwarding and S2 can maintain the
relationship with S1, at the same time, network topology between S3 and S4 will not be changed, furthermore,
it does not need to count routing again. All of these ensure the traffic forwarding and avoid routing shiver.
Layer 3 switch can be divided into GR restarter and GR helper according to its function in GR process. GR
restarter is layer 3 switch to occur the switchover between active master and standby master or restart