RFC 2439 (rfc2439) - Page 2 of 37
BGP Route Flap Damping
Alternative Format: Original Text Document
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
