BGP Nonstop Routing
BGP NSR is a unique, self-contained routing high-availability solution that extends IP high-availability deployments and benefits to the entire
edge. Currently BGP supports NSF (through BGP Graceful Restart) as part of the NSF/SSO high-availability offering in Cisco IOS Software.
BGP NSR extends the routing high-availability capabilities to the next level by “maintaining all necessary” BGP routing and session
information across a route-processor switchover.
Cisco support for BGP Nonstop Forwarding (also called Graceful Restart) follows the implementation specification described in the IETF proposed standard. According to this implementation, to achieve continuous packet forwarding the following conditions must be met:
• The NSF-capable router and the peer router must each agree to support BGP Graceful Restart.
• The peer router must not prematurely declare the NSF-capable router as unavailable.
• The peer router must not communicate any state change in the NSF-capable router to any of its peers. This avoids the networkwide detrimental effect on performance associated with the sudden failure of a router.
• The peer router must send BGP updates to help the restarting NSF-capable router to reacquire its BGP RIB.
• The peer router must signal the completion of the initial routing update by sending the End-of-RIB marker.
• In the interim (before the restarting NSF-capable router has reacquired the routing information), the peer router must mark any routes associated with the restarting router as "stale", but continue to use those routes for packet forwarding.
From:Cisco IOS Software High-Availability Enhancements for IP/MPLS Provider Edge
Cisco Nonstop Forwarding with Stateful Switchover Deployment Guide
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