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What Causes Route Flapping?

Level 14

What is Route Flapping?

Route flapping can be termed as the stream of fluctuating routing updates received by the routers on your network as they are routing traffic based on pre-defined routing policies. A route flap occurs when a router alternately advertises a destination network via one route then another, or when there’s an interface error on the router that alternates the availability of the router as up or down. When this happens repeatedly, the routing topology is distorted and it becomes difficult for the traffic-sending network device to determine the next route hop. The longer it takes to determine the next possible route path, it’ll lead to the network service latency or downtime.

Some level of route flap is unavoidable and is quite common. But when the flap frequency is very intensive and the destination router availability status keeps fluctuating, the routing topology would not converge and the traffic may be re-routed to other devices creating routing loops making the situation even worse.

What Causes Route Flapping?

The major reasons for route flap are:

  • Hardware errors
  • Software errors
  • Configuration errors (such as misconfigured Channel Service Units)
  • Intermittent errors in communications links
  • Unreliable connections

Route flapping is a common condition in the network when dynamic adaptive routing is used. This approach dynamically propagates information on topological changes to routers causing them to advertise or withdraw availability based on the topology changes. If the topology changes are too intensive, there will be a flap.

How to Control Route Flapping?

There are two ways to control route flapping:

#1 Route Dampening

Route dampening is a way of suppressing flapping routes so that they are "suppressed" instead of being advertised. To accomplish this, we define some criteria to identify poorly behaved routes. A route which is flapping gets a penalty for each flap. As soon as the cumulative penalty reaches a predefined "suppress−limit", the advertisement of the route will be suppressed. The penalty will be exponentially decayed based on a preconfigured "half−time". Once the penalty decreases below a predefined "reuse−limit", the route advertisement will be un−suppressed.[1]

Route dampening will be turned off by default. You can use the following commands to switch on and control route dampening for BGP protocol:

  • bgp dampening – this will turn on dampening
  • no bgp dampening – this will turn off dampening

#2 Route Aggregation

Route aggregation (or route summarization) is the process of limiting the visibility of topology details so that routing updates caused by topology changes do not reach the router. This process consolidates selected network routes into a single route advertisement and improves network stability by reducing unnecessary routing updates when a part of the network undergoes a change in topology.

Route flapping is one of the reasons the network faces latency and downtime, and network performance and VoIP call quality are impacted. SolarWinds Network Performance Monitor allows you to discover and view routing table information for monitored nodes, identify flapping routes, and create alerts for detected routing table changes. NPM supports RIP v2, OSPF v2 and v3, and BGP protocols for route monitoring.

Route Flapping.png

Learn More

Read this white paper to understand the importance of network route monitoring and ways to troubleshoot route errors.

Route Monitoring White Paper.png

6 Comments
Level 9

Is there a resource you can point me to that shows how to add alerts for OSPF routing table changes?

Level 17

NPM does have a built in alert for "Alert me when a neighbor goes down"

Level 15

Educational.

Level 10

This works well but what if it is flapping in between the Routing Neighbor poll, ours is set to 5 minutes, so each poll it's still established and the change is not detected but the flapping routes is increasing. Is there a way to setup an alert if a route flaps more than X amount of times?

We recently had an instance that a BGP neighbor still shows last change days ago yet the flapping routes shows it changed 12 times in the last 24 hours.

MVP
MVP

Very thorough information. Thanks.

One of the most frustrating parts of routing grrr.

About the Author
Vinod Mohan is a Senior Product Marketing Manager at DataCore Software. He has over a decade of experience in product, technology and solution marketing of IT software and services spanning application performance management, network, systems, virtualization, storage, IT security and IT service management (ITSM). In his current capacity at DataCore, Vinod focuses on communicating the value proposition of software-defined storage to IT teams helping them benefit from infrastructure cost savings, storage efficiency, performance acceleration, and ultimate flexibility for storing and managing data. Prior to DataCore, Vinod held product marketing positions at eG Innovations and SolarWinds, focusing on IT performance monitoring solutions. An avid technology enthusiast, he is a contributing author to many popular sites including APMdigest, VMblog, Cyber Defense Magazine, Citrix Blog, The Hacker News, NetworkDataPedia, IT Briefcase, IT Pro Portal, and more.