Reference Guide

Enabling Route Flap Dampening
When EBGP routes become unavailable, they “flap” and the router issues both WITHDRAWN and UPDATE
notices.
A flap is when a route:
• is withdrawn
• is readvertised after being withdrawn
• has an attribute change
The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP
process. To minimize this instability, you may configure penalties (a numeric value) for routes that flap.
When that penalty value reaches a configured limit, the route is not advertised, even if the route is up. In
Dell Networking OS, that penalty value is 1024. As time passes and the route does not flap, the penalty
value decrements or is decayed. However, if the route flaps again, it is assigned another penalty.
The penalty value is cumulative and penalty is added under following cases:
• Withdraw
• Readvertise
• Attribute change
When dampening is applied to a route, its path is described by one of the following terms:
• history entry — an entry that stores information on a downed route
• dampened path — a path that is no longer advertised
• penalized path — a path that is assigned a penalty
To configure route flap dampening parameters, set dampening parameters using a route map, clear
information on route dampening and return suppressed routes to active state, view statistics on route
flapping, or change the path selection from the default mode (deterministic) to non-deterministic, use
the following commands.
• Enable route dampening.
CONFIG-ROUTER-BGP mode
bgp dampening [half-life | reuse | suppress max-suppress-time] [route-map
map-name]
Enter the following optional parameters to configure route dampening parameters:
– half-life: the range is from 1 to 45. Number of minutes after which the Penalty is decreased.
After the router assigns a Penalty of 1024 to a route, the Penalty is decreased by half after the half-
life period expires. The default is 15 minutes.
– reuse: the range is from 1 to 20000. This number is compared to the flapping route’s Penalty
value. If the Penalty value is less than the reuse value, the flapping route is once again advertised
(or no longer suppressed). Withdrawn routes are removed from history state. The default is
750.
– suppress: the range is from 1 to 20000. This number is compared to the flapping route’s Penalty
value. If the Penalty value is greater than the suppress value, the flapping route is no longer
advertised (that is, it is suppressed). The default is 2000.)
– max-suppress-time: the range is from 1 to 255. The maximum number of minutes a route can
be suppressed. The default is four times the half-life value. The default is 60 minutes.
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Border Gateway Protocol IPv4 (BGPv4)