routedamping

Route dampening is a technique used in IP routing, most commonly with BGP, to reduce instability caused by frequent route flapping. The core idea is to assign a penalty to a route whenever it is withdrawn or re-advertised, and to suppress routes that flap too often.

Mechanism and operation: Each time a route changes state (such as withdrawal or re-advertisement), a route’s penalty is increased by a fixed amount. If the accumulated penalty exceeds a configured reuse threshold, the route is dampened and temporarily suppressed from being advertised to peers. While suppressed, the route can only be reintroduced if its penalty decays over time. Penalties decay exponentially according to a half-life time constant, so a non-flapping period causes the penalty to drop toward zero and eventually allow the route to be re-advertised. Suppression limits and decay parameters are chosen per router and per prefix.

Configuration and parameters: Typical dampening setups include a penalty increment for each flap, a reuse threshold that triggers suppression, a maximum penalty or suppress limit, and a half-life decay time. Some implementations also distinguish between different types of flaps or apply different values for external versus internal routes. Dampening is maintained on a per-prefix basis and is local to the router performing the dampening.

Scope and considerations: Route dampening aims to stabilize routing tables and reduce churn, but it can delay legitimate route changes and slow convergence after topology events. It may interfere with rapid scale-up in dynamic environments and can lead to suboptimal path selection if suppressions persist. Therefore, many operators tune or disable dampening based on network characteristics and policy.

History and status: Route dampening was described in RFC 2439 as a mechanism for stabilizing dynamic routing environments. It remains a configurable option in many router platforms, though its use is often carefully weighed against potential impacts on convergence and reachability.