From CCIE Study Wiki

BGP NSF Awareness allows BGP peers of the failing router to retain the routing information that is advertised by the failing router and to continue to use this info until the router has returned to normal operating behavior.

Theory

NSF or Non-Stop Forwarding, allows the routers data plane to continue to forward traffic while the routing process in the control plane reboots. Cisco IOS relies on CEF to handle the forwarding while the routing process reboots. Also referred to as graceful restart

Devices participating in BGP NSF must be configured for graceful restart. After configuring, existing sessions must be reset (clear ip bgp * or reload) before graceful restart capabilities will be exchanged.

BGP marks routes from a router undergoing restart as stale but still uses them.

Commands

• bgp graceful-restart [restart-time seconds] [stalepath-time seconds] (router) - Enables NSF forwarding awareness in BGP/

Default Settings

• Graceful restart is not enabled by default.
• The restart-time, or time peer routers will wait to delete stale routes before a BGP open message is received is 120 seconds.
• The stalepath-time, or how long a router will wait to delete stale routes after an end of record (EOR) message is received is 360 seconds

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• BGP graceful restart cannot support to routers undergoing a NSF restart at the same time.

Online Resources

• Cisco IOS IP routing protocol configuration guide on BGP NSF awareness.

• Cisco whitepaper on BGP NSF awareness support.

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