BGP Fall Over


BGP is very slow converging compared to IGP protocols. These features aid the convergence process by shortening the time a peer is detected as down. This is usually the role of the hold timer and its expiration. The fast-external-fallover feature relies on directly connected link, the fast peering deactivation relies on a route to track the BGP peer address in the IGP routing table which is used to connect to a peer (in eBGP multihop sessions) to speed up convergence. Selective address tracking improves regular fast peering deactivation, by specifying a route-map to use a specific route instead of being allowed by a summary route.

Fast External Fallover

  • Immediately reset session if a link to a directly connected external peer goes down.
  • Does not work too well for NBMA links and Metro Ethernet as the directly connected link usually don't go down.
  • Enabled by default can be changed with no bgp fast-external-fallover.
  • This feature is also controlled per interface with ip bgp fast-external-fallover (permit | deny).
  • This feature does not apply to iBGP nor to multi-hop eBGP.

Fast Peering Deactivation

  • Session is tear down when the router loses the IGP route it uses to reach that peer.
  • This applies to iBGP and eBGP multihop.
  • This feature is configured by neighbor using the command neighbor (ip) fall-over
  • Fast Peering Deactivation won't work if a summary route or default routes are present in IGP toward the bgp neighbors address. One way is to advertise the default route in BGP (which can not be used as a possible route to a BGP peer) or use the next described feature Selective Address Rracking for Fast Session Deactivation.

Selective Address Tracking for Fast Session Deactivation

  • Featured used as an enhancement where a summary matches a BGP peer route. In this case it will never go down and router will have to wait the full holddown timer to drop the session.
  • Enhances the Fast Peering Deactivation feature by looking for specific routes to reach the peer (instead of allowing summary or default routes).
  • The route-map should select a specific route used to reach the peer, like a loopback /32.
  • The only allowed match statements are match ip address and match source-protocol
  • If the route-map returns a deny condition session is brought down.

Relevant Commands

no bgp fast-external-fallover

Control using directly connected interface for peer session deactivation.

R1(config-router)#                no bgp fast-external-fallover

ip bgp fast-external-fallover (permit | deny)

Control using directly connected interface for peer session deactivation on a interface level. The no command has no effect.

R1(config-if)#                     ip bgp fast-external-fallover (permit | deny)

neighbor fall-over

Fast Peering Deactivation used IGP routes to peers to detect session down.

R1(config-router)#                neighbor (ip) fall-over

neighbor fall-over route-map

Selective Address Tracking for Fast Session Deactivation uses IGP routes, but in addition can specify which routes, bypassing defaults and summary routes.

R1(config-router)#                neighbor (ip) fall-over route-map (RMAP)

debug ip bgp rib-filter

#debug ip bgp rib-filter

Additional Resources

Designing Fast Converging BGP Networks NIL - Great discussion on the progress of these features.


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