When routers or hardware along a route go offline and back online that is called a route flap. Flapping is the term if these outages continue, especially if they occur frequently.

Route flap is a problem in BGP because each time a peer or a route goes down, all the peer routers that are connected to that out-of-service router advertise the change in their routing tables which creates a lot of administration traffic on the network. And the same traffic happens again when that router comes back online. If the problem is something like a faulty network cable that wobbles on and offline every 10 seconds, there could easily be overwhelming amounts of routing updates sent out unnecessarily.

Another possible reason for route flap occurs with multiple FortiGate units in HA mode. When an HA cluster fails over to the secondary unit, other routers on the network may see the HA cluster as being offline resulting in route flap. While this doesn’t occur often, or more than once at a time, it can still result in an interruption in traffic which is unpleasant for network users. The easy solution for this problem is to increase the timers on the HA cluster, such as TTL timers, so they do not expire during the failover process. Also configuring graceful restart on the HA cluster will help with a smooth failover.

The first method of dealing with route flap should be to check your hardware. If a cable is loose or bad, it can easily be replaced and eliminate the problem. If an interface on the router is bad, either avoid using that interface or swap in a functioning router. If the power source is bad on a router, either replace the power supply or use a power conditioning backup power supply. These quick and easy fixes can save you from configuring more complex BGP options. However if the route flap is from another source, configuring BGP to deal with the outages will ensure your network users uninterrupted service.

Some methods of dealing with route flap in BGP include: