Chapter 2 Advanced Routing for FortiOS 5.0 : Border Gateway Protocol (BGP) : Troubleshooting BGP : Route flap : Holddown timer
  
Holddown timer
The first line of defence to a flapping route is the hold down timer. This timer reduces how frequently a route going down will cause a routing update to be broadcast.
Once activated, the holddown timer won’t allow the FortiGate unit to accept any changes to that route for the duration of the timer. If the route flaps five times during the timer period, only the first outage will be recognized by the FortiGate unit — for the duration of the other outages there will be no changes because the Fortigate unit is essentially treating this router as down. After the timer expires, if the route is still flapping it will happen all over again.
Even if the route isn’t flapping — if it goes down, comes up, and stays back up — the timer still counts down and the route is ignored for the duration of the timer. In this situation the route will be seen as down longer than it really is, but there will be only the one set of route updates. This is not a problem in normal operation because updates are not frequent.
Also the potential for a route to be treated as down when it is really up can be viewed as a robustness feature. Typically you do not want most of your traffic being routed over an unreliable route. So if there is route flap going on, it is best to avoid that route if you can. This is enforced by the holddown timer.
How to configure the holddown timer
There are three different route flapping situations that can occur: the route goes up and down frequently, the route goes down and back up once over a long period of time, or the route goes down and stays down for a long period of time. These can all be handled using the holddown timer.
For example, your network has two routes that you want to set the holddown timer for. One is your main route ( to 10.12.101.4) that all your Internet traffic goes through, and it can’t be down for long if its down. The second is a low speed connection to a custom network that is used infrequently ( to 10.13.101.4). The holddown timer for the main route should be fairly short, lets say 60 seconds instead of the default 180 seconds. The second route timer can be left at the default or even longer since it is rarely used. In your BGP configuration this looks like:
config router bgp
config neighbor
edit 10.12.101.4
set holddown-timer 60
next
edit 10.13.101.4
set holddown-timer 180
next
end
end