Advanced Cisco BGP features: NSF

Below topology was used for this post, and all the configuration happened on two Cisco CSR1000v

BGP Nonstop Forwarding

  • During normal NSF operation, CEF on the active RP synchronizes its current FIB and adjacency databases with the FIB and adjacency databases on the standby RP
  • While switching over, the traffic is depended on CEF, once the routing protocol is converged, FIB will be updated
  • RIB repopulating happens prefix-by-prefix, thus the same for FIB and adjacency table
  • For BGP NSF, graceful-restart needs to be configured on both ends of a BGP session. Although one end could be only NSF-aware (not SSO capable)
  • BFD can’t be enabled simultaneously with NSF for BGP
  • SSO is not integrated into EIGRP, hence only NSF awareness is supported

By entering  bgp graceful-restart some default parameters will be also configured:

The optional restart-time determine how long peer routers will wait to delete stale routes before a BGP open message is received.
The optional stalepath-time determine how long a router will wait before deleting stale routes after an end of record (EOR) message is received from the restarting router.

When graceful-restart is not enabled:

In case this feature is configured after session establishments, BGP sessions should be cleared!

CSR2(config-router)#bgp graceful-restart    
All BGP sessions must be reset to take the new GR config
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Author: Mo Moghaddas

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