In which two ways does the BGP graceful restart capability preserve prefix information during a restarting router? (Choose two.)
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BGP (Border Gateway Protocol) graceful restart capability is a feature that helps in preserving prefix information during a restarting router. The primary purpose of the graceful restart capability is to minimize the impact of a router restart on the overall network.
The two ways that BGP graceful restart capability preserves prefix information during a restarting router are:
The restarting router does not remove any stale prefixes after the timer for stale entries expires: When a router restarts, it loses all the routes in its forwarding table. The BGP graceful restart feature helps in preserving the prefix information by allowing the restarting router to continue forwarding traffic for the stale prefixes that were already known before the restart. This is achieved by the restarting router marking the stale prefixes with a "stale" flag and keeping them in the forwarding table until the peer router sends updates with new information about those prefixes. During this time, the restarting router does not remove any stale prefixes, even after the timer for stale entries expires.
The peer router sends an end-of RIB message to the restarting router: When the peer router detects that the restarting router is back online, it sends an end-of-RIB (EoR) message to the restarting router. This message signals to the restarting router that the peer router has finished sending all the BGP routing information and that the restarting router can resume normal operation. The restarting router then updates its forwarding table with the new prefix information received from the peer router.
To summarize, the BGP graceful restart capability helps in preserving prefix information during a router restart by allowing the router to continue forwarding traffic for stale prefixes and by providing a mechanism for the peer router to signal when it has finished sending new routing information.