As we have already seen that(OSPF High Availability with SSO,NSF and NSR) there are two different mechanism to prevent routing protocol re-convergence during a processor switch-over. One is Graceful Restart(Non Stop Forwarding) and another is Non Stop Routing(NSR). Graceful Restart and Non Stop Routing allows for the forwarding of data packets to continue along known routes. By using Graceful Restart routing protocol information is being restored while using Non Stop Routing, routing protocol information is being refreshed.
BGP Graceful Restart for NSF
• BGP Graceful Restart is described in RFC 4724
• BGP has been enhanced with NSF-capability and awareness
• Routers running these protocols can detect a switchover and take the necessary actions to continue forwarding network traffic and to recover route information from the peer devices
• NSF Aware
– A router is NSF-aware if it is running NSF-compatible software.
• NSF Capable
– A router is NSF-capable if it has been configured to support NSF; therefore, it would rebuild routing information from NSF-aware or NSF-capable neighbors.
• BGP support for NSF requires that neighbor routers are NSF-aware or NSF-capable
• A router that is NSF-aware functions like a router that is NSF-capable with one exception: an NSF-aware router is incapable of performing an SSO operation
• A router that is NSF-aware is capable of maintaining a peering relationship with a NSF-capable neighbor during a NSF SSO operation, as well as holding routes for this neighbor during the SSO operation
• NSF awareness for BGP is not enabled by default as BGP uses TCP connection. It can be enabled by using bgp graceful-restart under BGP process.