Route Flap Damping Exacerbates Internet Routing Convergence

Zhuoqing Morley Mao, Ramesh Govindan1, and George Varghes2
(Professor Randy H. Katz)
California MICRO Program, Ericsson, Nokia, Siemens, and Sprint

Route flap damping is a widely deployed mechanism in core routers that limits the widespread propagation of unstable BGP routing information. Originally designed to suppress route changes caused by link flaps, flap damping attempts to distinguish persistently unstable routes from routes that occasionally fail. It is considered to be a major contributor to the stability of the Internet routing system.

We show in this project that, surprisingly, route flap damping can significantly exacerbate the convergence times of relatively stable routes. For example, a route to a prefix that is withdrawn exactly once and re-announced can be suppressed for up to an hour (using the current RIPE recommended damping parameters). We show that such abnormal behavior fundamentally arises from the interaction of flap damping with BGP path exploration during route withdrawal. We study this interaction using a simple analytical model, and understand through the use of simulations the impact of various BGP parameters on its occurrence. Finally, we outline a preliminary proposal to modify a route flap damping scheme (based on ignoring monotonic route changes that flap damping triggers) that removes the undesired interaction in all the topologies we studied.

1Professor, USC
2Professor, UC San Diego

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