What are the causes and consequences of route flapping in a network?

1 Causes of route flapping

Route flapping can be caused by various factors, such as hardware failures, misconfigurations, link failures, network congestion, routing protocol bugs, or malicious attacks. For example, if a router has a faulty interface or cable that connects to another router, it may alternately report that the link is up and down, causing route flapping. Similarly, if a router is configured with incorrect or inconsistent routing policies, it may advertise or withdraw routes based on incorrect criteria, causing route flapping. Another common cause of route flapping is a routing loop, which occurs when a packet is forwarded between two or more routers indefinitely, without reaching its destination. This can happen when routers have conflicting or outdated routing information, or when there is a lack of route summarization or filtering.

2 Consequences of route flapping

Route flapping can have negative consequences for both the network and the applications that rely on it. For the network, route flapping can increase the bandwidth consumption, CPU utilization, and memory usage of the routers, as they have to process and propagate frequent route updates. This can reduce the network performance, reliability, and scalability, and increase the risk of network failures or outages. For the applications, route flapping can cause packet loss, latency, jitter, and retransmissions, as the packets may be dropped or delayed by the routers, or sent through suboptimal paths. This can degrade the quality of service, user experience, and security of the applications, especially those that are sensitive to network conditions, such as voice, video, or online gaming.

3 Prevention of route flapping

Route flapping can be prevented or mitigated by applying some best practices and techniques in network design, configuration, and monitoring. For example, network administrators can use redundant links, backup routers, and failover mechanisms to avoid single points of failure and ensure high availability. They can also use consistent and appropriate routing policies, protocols, and parameters to avoid misconfigurations and conflicts. Additionally, they can use route summarization, filtering, aggregation, and dampening to reduce the number and frequency of route updates and prevent routing loops. Furthermore, they can use network monitoring tools and protocols to detect and troubleshoot route flapping and other network issues.

4 Route dampening

Route dampening is a technique that can help reduce the impact of route flapping by suppressing or penalizing unstable routes. It works by assigning a penalty value to each route based on the number and frequency of its updates. If the penalty value exceeds a predefined threshold, the route is suppressed or marked as unreachable. The penalty value decays exponentially over time, and if it falls below another threshold, the route is unsuppressed or marked as reachable again. Route dampening can help improve network stability and performance by preventing routers from sending or receiving excessive route updates. However, it can also introduce some drawbacks, such as delayed convergence, increased complexity, and false positives.

6 Routing loop vs route flapping

Routing loop and route flapping are two related but distinct concepts in network routing. A routing loop is a situation where a packet is forwarded between two or more routers indefinitely, without reaching its destination. A routing loop can cause route flapping, which is a situation where a router repeatedly advertises and withdraws a route to a destination. However, route flapping can also occur without a routing loop, due to other factors such as hardware failures, misconfigurations, link failures, network congestion, routing protocol bugs, or malicious attacks. Routing loop and route flapping can both cause network instability, performance degradation, and resource exhaustion.