RFC 2439 (rfc2439) - Page 2 of 37

BGP Route Flap Damping

RFC 2439                 BGP Route Flap Damping            November 1998


   o  minimal addition space and computational overhead

   An excessive rate of update to the advertised reachability of a
   subset of Internet prefixes has been widespread in the Internet.
   This observation was made in the early 1990s by many people involved
   in Internet operations and remains the case.  These excessive updates
   are not necessarily periodic so route oscillation would be a
   misleading term.  The informal term used to describe this effect is
   "route flap".  The techniques described here are now widely deployed
   and are commonly referred to as "route flap damping".

1 Overview

   To maintain scalability of a routed internet, it is necessary to
   reduce the amount of change in routing state propagated by BGP in
   order to limit processing requirements.  The primary contributors of
   processing load resulting from BGP updates are the BGP decision
   process and adding and removing forwarding entries.

   Consider the following example.  A widely deployed BGP implementation
   may tend to fail due to high routing update volume.  For example, it
   may be unable to maintain it's BGP or IGP sessions if sufficiently
   loaded.  The failure of one router can further contribute to the load
   on other routers.  This additional load may cause failures in other
   instances of the same implementation or other implementations with a
   similar weakness.  In the worst case, a stable oscillation could
   result.  Such worse cases have already been observed in practice.

   A BGP implementation must be prepared for a large volume of routing
   traffic.  A BGP implementation cannot rely upon the sender to
   sufficiently shield it from route instabilities.  The guidelines here
   are designed to prevent sustained oscillations, but do not eliminate
   the need for robust and efficient implementations.  The mechanisms
   described here allow routing instability to be contained at an AS
   border router bordering the instability.

   Even where BGP implementations are highly robust, the performance of
   the routing process is limited.  Limiting the propagation of
   unnecessary change then becomes an issue of maintaining reasonable
   route change convergence time as a routing topology grows.

2 Methods of Limiting Route Advertisement

   Two methods of controlling the frequency of route advertisement are
   described here.  The first involves fixed timers.  The fixed timer
   technique has no space overhead per route but has the disadvantage of
   slowing route convergence for the normal case where a route does not
   have a history of instability.  The second method overcomes this



Villamizar, et. al.         Standards Track