Tuning BGP Performance - BGP Design and Implementation - page 88

receiving BGP router. The receiving BGP router accepts the new session and closes the old
TCP connection. The receiving router does not send a BGP NOTIFICATION, as per the
normal behavior.
The receiving BGP router sends the BGP-GR capability to the restarting BGP router, with
the Restart State set to 0, unless the receiving BGP router also is reset. The receiving BGP
router receives a BGP-GR capability from the restarting BGP router with a Restart State of 1.
This triggers the receiving BGP router to send the initial routing update, followed by the
end-of-RIB marker.
The receiving router maintains the “stale” routing information until one of three events
occurs:
• The receiving router detects that forwarding state is not kept on the restarting router
via the Forwarding State bit received in the BGP-GR capability.
• The receiving router receives the end-of-RIB marker from the restarting router.
• The receiving router has a failsafe timer expire that ensures that the “stale”
information does not remain in use.
An important distinction to make is between the first BGP peering session, where the BGP-
GR capability is established, and the first restarting session. The first session initiated with
the BGP-GR capability tells the peers that they should maintain its prefixes if it goes down.
The capability is received for the second time after the restart. This reestablishes that the
peers should maintain the BGP prefix information for future restarts. It also indicates that
the forwarding state was maintained over the restart, preventing an impact on data traffic.
The BGP NSF feature is enabled globally and not on a peer-by-peer basis. It is enabled with
the following configuration:
router bgp ASN
bgp graceful-restart
The BGP restart time is configured with the command bgp graceful-restart restart-time
value with a granularity of seconds.
You can also configure the stale path timer with a granularity in seconds for BGP NSF. The
stale path timer determines how long a router holds onto the paths received from a grace-
fully restarted peer. You configure this timer with the command bgp graceful-restart stale-
path-time value .
The BGP NSF feature can be verified using the output from show ip bgp neighbors . You
also can verify that the graceful restart capability has been advertised and received. The
BGP NSF feature was integrated into Releases 12.0(22)S and 12.2.15T.