Editing Border Gateway Protocol (section)

== Stability ==

The routing tables managed by a BGP implementation are adjusted continually to reflect actual changes in the network, such as links or routers going down and coming back up. In the network as a whole, it is normal for these changes to happen almost continuously, but for any particular router or link, changes are expected to be relatively infrequent. If a router is misconfigured or mismanaged then it may get into a rapid cycle between down and up states. This pattern of repeated withdrawal and re-announcement known as [[route flapping]] can cause excessive activity in all the other routers that know about the cycling entity, as the same route is continually injected and withdrawn from the routing tables. The BGP design is such that delivery of traffic may not function while routes are being updated. On the Internet, a BGP routing change may cause outages for several minutes.

A feature known as ''route flap damping'' ({{IETF RFC|2439}}) is built into many BGP implementations in an attempt to mitigate the effects of route flapping. Without damping, the excessive activity can cause a heavy processing load on routers, which may in turn delay updates on other routes, and so affect overall routing stability. With damping, a route's flapping is [[exponential decay|exponentially decayed]]. At the first instance when a route becomes unavailable and quickly reappears, damping does not take effect, so as to maintain the normal fail-over times of BGP. At the second occurrence, BGP shuns that prefix for a certain length of time; subsequent occurrences are ignored exponentially longer. After the abnormalities have ceased and a suitable length of time has passed for the offending route, prefixes can be reinstated with a clean slate. Damping can also mitigate [[denial-of-service attack]]s.

It is also suggested in RFC 2439{{rp|Section 4}} that route flap damping is a feature more desirable if implemented to Exterior Border Gateway Protocol Sessions (eBGP sessions or simply called exterior peers) and not on Interior Border Gateway Protocol Sessions (iBGP sessions or simply called internal peers). With this approach when a route flaps inside an autonomous system, it is not propagated to the external ASs{{snd}} flapping a route to an eBGP will cause a chain of flapping for the particular route throughout the backbone. This method also successfully avoids the overhead of route flap damping for iBGP sessions.

Subsequent research has shown that flap damping can actually lengthen convergence times in some cases, and can cause interruptions in connectivity even when links are not flapping.<ref>{{cite web|url=http://web.eecs.umich.edu/~zmao/Papers/sig02.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://web.eecs.umich.edu/~zmao/Papers/sig02.pdf |archive-date=2022-10-09 |url-status=live|title=Route Flap Damping Exacerbates Internet Routing Convergence|date=November 1998}}</ref><ref>{{cite web | url = http://www.cs.arizona.edu/~bzhang/paper/05-icdcs-dtimer.pdf | last = Zhang | first = Beichuan | title = Timer Interaction in Route Flap Damping | access-date = 2006-09-26 |author2=Pei Dan |author3=Daniel Massey |author4=[[Lixia Zhang]] |date=June 2005 | work = IEEE 25th [[International Conference on Distributed Computing Systems]] | quote = We show that the current damping design leads to the intended behavior only under persistent route flapping. When the number of flaps is small, the global routing dynamics deviates significantly from the expected behavior with a longer convergence delay. }}</ref> Moreover, as backbone links and router processors have become faster, some network architects have suggested that flap damping may not be as important as it used to be, since changes to the routing table can be handled much faster by routers.<ref>{{cite news|url=https://tools.ietf.org/html/rfc2439 |title=BGP Route Flap Damping |date=November 1998 |publisher=Tools.ietf.org|last1=Villamizar |first1=Curtis |last2=Chandra |first2=Ravi |last3=Govindan |first3=Ramesh |newspaper=Ietf Datatracker }}</ref> This has led the RIPE Routing Working Group to write, "With the current implementations of BGP flap damping, the application of flap damping in ISP networks is NOT recommended. ... If flap damping is implemented, the ISP operating that network will cause side-effects to their customers and the Internet users of their customers' content and services ... . These side-effects would quite likely be worse than the impact caused by simply not running flap damping at all."<ref>{{cite web|url=http://www.ripe.net/ripe/docs/ripe-378 |title=RIPE Routing Working Group Recommendations On Route-flap Damping |publisher= RIPE Network Coordination Centre |date=2006-05-10 | access-date = 2013-12-04 }}</ref> Improving stability without the problems of flap damping is the subject of current research.<ref>{{cite news|url=http://tools.ietf.org/html/draft-ymbk-rfd-usable |title=draft-ymbk-rfd-usable-02 - Making Route Flap Damping Usable |newspaper=Ietf Datatracker |publisher=Tools.ietf.org | access-date = 2013-12-04 }}</ref>{{update inline|reason=Ref indicates research was in progress in 2011.|date=December 2022}}