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[ScreenOS] How to adjust BGP flap damping?

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Article ID: KB16142 KB Last Updated: 11 Mar 2019Version: 4.0
Summary:

BGP flap dampening is a feature used to reduce the propagation of flapping routes without affecting convergence time of non flapping routes. By reducing the propagation of flapping routes, the utilization of routers running BGP can be reduced and hence increasing the network stability.

‚ÄčThis article describes a hidden command to adjust the Border Gateway Protocol (BGP) flap damping parameters on ScreenOS devices 

Symptoms:

ScreenOS supports a BGP flap damping feature, but it is currently hidden. The default values are:

get vrouter trust protocol bgp flap-damping
Route Flap Damping            : disabled
Reuse                         : 1024
Suppress Limit                : 2048
Max Penalty                   : 4096
Reachable Half Life           : 5 minutes 0 seconds
Unreachable Half Life         : 15 minutes 0 seconds
Maximum Reachable Hold Time   : 15 minutes 0 seconds
Maximum Unreachable Hold Time : 30 minutes 0 seconds
Decay factor : 4

The penalty value for each route flap is 1024.

Cause:

BGP flapping can occur when you have an unstable peer, rapidly cycling between states.

Solution:

There is a hidden command to adjust the BGP flap damping parameters:

set vrouter trust protocol bgp flap-damping penalty-limit <reuse limit> <suppress limit> <penalty ceiling>

Example:

set vrouter trust protocol bgp flap-damping penalty-limit 500 1000 2000

get vrouter trust protocol bgp flap-damping
Route Flap Damping            : disabled
Reuse                         : 500
Suppress Limit                : 1000
Max Penalty                   : 2000
Reachable Half Life           : 5 minutes 0 seconds
Unreachable Half Life         : 15 minutes 0 seconds
Maximum Reachable Hold Time   : 15 minutes 0 seconds
Maximum Unreachable Hold Time : 30 minutes 0 seconds
Decay factor : 4

Penalty:
An incremented numeric value that is assigned to a route each time it flaps. A route is penalized 1000 every time it flaps. A route is considered to have flapped when we receive a WITHDRAW and then an UPDATE for a route. If we drop and regain the adjacency with a BGP peer we do not assign a penalty to the routes learned from that peer.

Suppress limit:
A numeric value that is compared with the penalty. If the penalty is greater than the suppress limit, the route is suppressed. .

Reuse limit:
A configurable numeric value that is compared with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up will no longer be suppressed.

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