Understanding Damping Parameters
BGP route flapping describes the situation in which BGP systems send an excessive number of update messages to advertise network reachability information. BGP flap damping is a method of reducing the number of update messages sent between BGP peers, thereby reducing the load on these peers, without adversely affecting the route convergence time for stable routes.
Flap damping reduces the number of update messages by marking routes as ineligible for selection as the active or preferable route. Marking routes in this way leads to some delay, or suppression, in the propagation of route information, but the result is increased network stability. You typically apply flap damping to external BGP (EBGP) routes (routes in different ASs). You can also apply flap damping within a confederation, between confederation member ASs. Because routing consistency within an AS is important, do not apply flap damping to internal BGP (IBGP) routes. (If you do, it is ignored.)
There is an exception that rule. Starting in Junos OS Release 12.2, you can apply flap damping at the address family level. In a Junos OS Release 12.2 or later installation, when you apply flap damping at the address family level, it works for both IBGP and EBGP.
By default, route flap damping is not enabled. Damping is applied to external peers and to peers at confederation boundaries.
When you enable damping, default parameters are applied, as summarized in Table 1.
Table 1: Damping Parameters
Decay half-life—Number of minutes after which an arbitrary value is halved if a route stays stable.
1 through 45
Maximum hold-down time for a route, in minutes.
1 through 720
Reuse threshold—Arbitrary value below which a suppressed route can be used again.
1 through 20,000
Cutoff (suppression) threshold—Arbitrary value above which a route can no longer be used or included in advertisements.
1 through 20,000
To change the default BGP flap damping values, you define actions by creating a named set of damping parameters and including it in a routing policy with the damping action. For the damping routing policy to work, you also must enable BGP route flap damping.