This article will explain about the working of BGP Dampening on Junos with the help of explaining its different parameter values

What is BGP Dampening?

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. In simple, BGP route flap damping helps to diminish route instability caused by routes being repeatedly withdrawn and readvertised when a link is intermittently failing. By default, route flap damping is not enabled.

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.

How it works?

Below are the Dampening parameters:

[edit policy-options]

damping name {

   disable;

   half-life minutes;

   max-suppress minutes;

   reuse number;

   suppress number;

}

To understand how to configure these parameters, you need to understand how damping suppresses routes. How long a route can be suppressed is based on a figure of merit, which is a value that correlates to the probability of future instability of a route. Routes with higher figure-of-merit values are suppressed for longer periods of time. The figure-of-merit value decays exponentially over time.

A figure-of-merit value of zero is assigned to each new route. The value is increased each time the route is withdrawn or readvertised, or when one of its path attributes changes. With each incident of instability, the value increases as follows:

Route is withdrawn—1000

Route is readvertised—1000

Route’s path attributes change—500

Note: 

Other vendors’ implementations for figure-of-merit increase the value only when a route is withdrawn. The Junos OS implementation for figure-of-merit increases the value for both route withdrawal and route readvertisement. To accommodate other implementations for figure-of-merit, multiply the reuse and suppress threshold values by 2.

When a route’s figure-of-merit value reaches a particular level, called the cutoff or suppression threshold, the route is suppressed. If a route is suppressed, the routing table no longer installs the route into the forwarding table and no longer exports this route to any of the routing protocols.

By default, a route is suppressed when its figure-of-merit value reaches 3000, as Junos is having a default penalty value of 1000, which will get added with each flap and by default the supress value is 3000. To modify this default, include the suppress option at the [edit policy-options damping name] hierarchy level.

For Example:

If a route has flapped, but then becomes stable so that none of the incidents listed previously occur within a configurable amount of time, the figure-of-merit value for the route decays exponentially. The default half-life is 15 minutes. For example, for a route with a figure-of-merit value of 1500, if no incidents occur, its figure-of-merit value is reduced to 750 after 15 minutes and to 375 after another 15 minutes. To modify the default half-life, include the half-life option at the [edit policy-options damping name] hierarchy level.

Once the penalty is below the reuse limit of 750, the route can be used again and advertised to other BGP routers. When the penalty is below 50% of the reuse limit, the penalty is removed from the route.

The maximum suppress limit ensures that a route won’t be dampened forever. A route’s figure-of-merit value stops increasing when it reaches a maximum suppression threshold, which is determined based on the route’s suppression threshold level, half-life, reuse threshold, and maximum hold-down time.

To display figure-of-merit information, use the show policy damping command.

A route that has been assigned a figure of merit is considered to have a damping state. To display the current damping information on the routing device, use the show route detail command.

For more details regarding this, please find the below detailed document:

https://www.juniper.net/documentation/us/en/software/junos/routing-policy/topics/concept/policy-using-routing-policies-to-damp-bgp-route-flapping.html

https://www.juniper.net/documentation/us/en/software/junos/routing-policy/bgp/topics/concept/routing-policy-security-damping-parameter-understanding.html  

For configuring BGP Dampening check below KB:

https://supportportal.juniper.net/s/article/Configuring-BGP-Flap-Damping