During session startup, BGP (Border Gateway Protocol) peers indicate whether they have the graceful restart capability. When BGP restarts, the restarting router indicates during the new session startup that it is restarting and provides a restart-time value, which defines how long the receiving router will wait for the restart to complete.
During the restart, all pre-existing routes are marked stale. BGP receives new routes during the restart, but the Routing Information Base (RIB) is not updated and advertised until the restart is complete. An End of RIB message is sent when the restart is complete if the graceful restart feature is enabled.
An update-delay timer value determines how long the restarting switch will wait before updating the stale routes. If this timer expires before the restarting switch receives updates from the receiving routers, all stale routes are deleted. If the receiving routers provide updates before this timer expires, the time-stamps for any matching entries in the local RIB are preserved.
After the new BGP session is established, the new session uses the capabilities established with that session, which includes any updates to the graceful restart capability or timers.
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