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Note: A Routing Engine switchover event on a helper router that is in graceful wait state causes the router to drop the wait state and to propagate the adjacency's state change to the network. |
With routing protocols, any service interruption requires an affected router to recalculate adjacencies with neighboring routers, restore routing table entries, and update other protocol-specific information. An unprotected restart of a router can result in forwarding delays, route flapping, wait times stemming from protocol reconvergence, and even dropped packets. To alleviate this situation, graceful restart provides extensions to routing protocols. These protocol extensions define two roles for a router—restarting and helper. The extensions signal neighboring routers about a router undergoing a restart and prevent the neighbors from propagating the change in state to the network during a graceful restart wait interval. The main benefits of graceful restart are uninterrupted packet forwarding and temporary suppression of all routing protocol updates. Graceful restart enables a router to pass through intermediate convergence states that are hidden from the rest of the network.
When a router is running graceful restart and the router stops sending and replying to protocol liveliness messages (hellos), the adjacencies assume a graceful restart and begin running a timer to monitor the restarting router. During this interval, helper routers do not process an adjacency change for the router that they assume is restarting, but continue active routing with the rest of the network. The helper routers assume that the router can continue stateful forwarding based on the last preserved routing state during the restart.
If the router was actually restarting and is back up before the graceful timer period expires in all of the helper routers, the helper routers provide the router with the routing table, topology table, or label table (depending on the protocol), exit the graceful period, and return to normal network routing.
If the router does not complete its negotiation with helper routers before the graceful timer period expires in all of the helper routers, the helper routers process the router's change in state and send routing updates, so that convergence occurs across the network. If a helper router detects a link failure from the router, the topology change causes the helper router to exit the graceful wait period and to send routing updates, so that network convergence occurs.
To enable a router to undergo a graceful restart, you must include the graceful-restart statement at either the global [edit routing-options] hierarchy level or the hierarchy level for a specific protocol. When a routing session is started, a router that is configured with graceful restart must negotiate with its neighbors to support it when it undergoes a graceful restart. A neighboring router will accept the negotiation and support helper mode without requiring graceful restart to be configured on the neighboring router.
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Note: A Routing Engine switchover event on a helper router that is in graceful wait state causes the router to drop the wait state and to propagate the adjacency's state change to the network. |
Graceful restart is supported for the following protocols and applications:
For more information, see Graceful Restart Overview.
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