Graceful Restart OSPF

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OSPF and OSPF version 3—When a router enabled for OSPF graceful restart restarts, it retains routes learned prior to the restart in its forwarding table. The router does not allow new OSPF link-state advertisements (LSAs) to update the routing table. This router continues to forward traffic to other OSPF neighbors (or helper routers), and sends only a limited number of LSAs during the restart period. To re-establish OSPF adjacencies with neighbors, the restarting router must send a grace LSA to all neighbors. In response, the helper routers enter helper mode and send an acknowledgement back to the restarting router. Also, if there are no topology changes, the helper routers continue to advertise LSAs as if the restarting router had remained in continuous OSPF operation.

OSPF 和 OSPF v3---当一个路由器开启OSPF优雅重启启动，它保留最新学习的路由条目放到转发表中。之后路由不在接受OSPF lsa 来更新route table。这个路由器能继续传输数据给其他OSPF邻居，并且不发送LSA。在恢复建立建立ospf 邻居时，restarting router 必须发送一个 grace LSA给所有的邻居。这时helper router 进入helper mode 和发送一个ACK返回给restarting router 。同样，如果拓扑没有改变，restarting router 能良好的工作，helper router 将公告LSA。。When the restarting router receives replies from all the helper routers, the restarting router selects routes, updates the forwarding table, and discards the old routes. At this point, full OSPF adjacencies are re-established and the restarting router receives and processes OSPF LSAs as usual. When the helper routers no longer receive grace LSAs from the restarting router or the topology of the network changes, the helper routers also resume normal operation.
   当restarting router 收到helper router 的LSA，restarting router 挑选路由并更新 forwarding 表，并丢弃老的路由。这时，完全邻居状态的OSPF邻开始重建，并且restarrting router 收到并且处理LSA。 当 helps router 很长时间没有收到来自restarting router 的grace LSA 。或者拓扑发生变化，helps router 进入正常程序