Information Technology Reference
In-Depth Information
The default suppress limit is 2000; however, it takes three route flaps to trigger suppression.
The penalty assigned for each flap is 1000; however, the penalty begins to decay immedi-
ately. The decay between the first and second flaps keeps the penalty below the suppress
limit of 2000 until the third flap.
The penalty assigned to a route decays over time. When the penalty for the route drops
below the reuse limit, it is again advertised to peers. This mechanism allows well-behaved
routes to converge quickly; however, routes that exhibit instability are dampened until the
instability subsides.
Example 3-11 shows the route-dampening feature configuration.
Example 3-11
Route Dampening Configuration
router bgp 100
bgp dampening half-life reuse-limit suppress-limit maximum-suppress-time
The configuration of BGP dampening parameters can result in unexpected behavior if care
is not taken when determining the values. The maximum penalty that can be assigned to a
prefix is determined through a formula. If the maximum penalty is not larger than the sup-
press limit, prefixes will never achieve a high-enough penalty to be suppressed, effectively
rendering BGP dampening useless. The formula is as follows:
max-suppress-time
half-life
---------------------------------------------
max-penalty = reuse-limit * 2
The following is an example of BGP dampening parameters that will prove ineffective:
bgp dampening 30 750 3000 60
The suppress limit is 3000, and the maximum penalty assigned to a route is 3000. The
penalty for a route must exceed the suppress limit. In this case, the penalty is equal to the
suppress limit.
The following is an example of the default values for BGP dampening that are effective:
bgp dampening 15 750 2000 60
This results in a suppress limit of 2000 and a maximum penalty of 6000. Always check
parameters to ensure that they will actually engage the BGP dampening feature for flapping
routes.
The BGP dampening feature affects only external BGP routes. If BGP dampening were
applicable to internal prefixes, disparate dampening parameters could provide inconsistent
forwarding tables throughout the network. Dampening the prefixes at the edge removes
them from the internal network, effectively providing internal dampening. The BGP damp-
ening feature operates on routes on a per-path basis. If a prefix has two paths, and one is
dampened, the other prefix is still available and is advertised to BGP peers.
