routeflapping

Routeflapping, or route flapping, is the rapid and repeated change in the reachability of a network route caused by frequent updates and withdrawals in routing protocols. In practice, a prefix may be announced and then withdrawn multiple times within a short interval, causing instability in forwarding state across routers.

While commonly discussed in the context of BGP, route flapping can occur in any dynamic routing protocol, including OSPF and RIP. It is typically triggered by unstable links, flaky interfaces, misconfigurations, hardware faults, or rapid policy changes at neighboring networks.

Consequences include control-plane churn, increased CPU and memory usage on routers, and transient traffic disruption as convergence occurs.

Mitigation often centers on route dampening, a mechanism formalized in RFC 2439 (BGP Route Flap Damping). Dampening assigns a penalty to each flap and suppresses unstable routes for a period when the penalty exceeds a threshold. Parameters such as half-life, reuse time, and penalties are tuned to balance convergence speed and stability. Additional measures include fixing underlying instability, tighter filtering, and faster failure detection (for example, Bidirectional Forwarding Detection) to reduce the chance of repeated flaps.

History: Route dampening rose to prominence in the late 1990s as Internet routing grew unstable. While effective at reducing churn, it can delay legitimate changes and is not universally applied; operators may disable damping for stable prefixes or rely on alternative stability mechanisms, such as redundancy and careful traffic engineering.