Network Working Group C. Villamizar
Request for Comments: 2439 ANS
Category: Standards Track R. Chandra
BGP Route Flap Damping
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1998). All Rights Reserved.
A usage of the BGP routing protocol is described which is capable of
reducing the routing traffic passed on to routing peers and therefore
the load on these peers without adversely affecting route convergence
time for relatively stable routes. This technique has been
implemented in commercial products supporting BGP. The technique is
also applicable to IDRP.
The overall goals are:
o to provide a mechanism capable of reducing router processing load
caused by instability
o in doing so prevent sustained routing oscillations
o to do so without sacrificing route convergence time for generally
well behaved routes.
This must be accomplished keeping other goals of BGP in mind:
o pack changes into a small number of updates
o preserve consistent routing
Villamizar, et. al. Standards Track [Page 1]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".
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 [Page 2]RFC 2439 BGP Route Flap Damping November 1998