Configuring Flap Damping Parameters

To define damping parameters, include the damping statement:

damping name { 

    disable;

    half-life minutes;

    max-suppress minutes;

    reuse number;

    suppress number;

} 

You can include this statement at the following hierarchy levels:

• [edit policy-options]
• [edit logical-routers logical-router-name policy-options]

The name identifies the group of damping parameters. It can contain letters, numbers, and hyphens (-) and can be up to 255 characters. To include spaces in the name, enclose the entire name in quotation marks (double quotes).

You can specify one or more of the damping parameters described in Table 25.

Table 25: Damping Parameters

Damping Parameter	Description	Default	Possible Values
half-life minutes	Decay half-life, in minutes	15 minutes	1 through 45 minutes
max-suppress minutes	Maximum hold-down time, in minutes	60 minutes	1 through 720 minutes
reuse	Reuse threshold	750 (unitless)	1 through 20,000 (unitless)
suppress	Cutoff (suppression) threshold	3000 (unitless)	1 through 20,000 (unitless)

If you do not specify one or more of the damping parameters, the default value of the parameter is used.

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 The value is increased each time the route is withdrawn or readvertised, or when one of its path attributes changes.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• Route is withdrawn—1000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  NOTE: Other vendors' implementations for figure-of-merit increase the value only when a route is withdrawn.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 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. To modify this default, include the suppress option at the [edit policy-options damping name] hierarchy level.

If a route has flapped, but then becomes stable so that none of the incidents listed above 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.

A suppressed route becomes reusable when its figure-of-merit value decays to a value below a reuse threshold, thus allowing routes that experience transient instability to once again be considered valid. The default reuse threshold is 750. When the figure-of-merit value passes below the reuse threshold, the route once again is considered usable and can be installed in the forwarding table and exported from the routing table. To modify the default reuse threshold, include the reuse option at the [edit policy-options damping name] hierarchy level.

The maximum suppression time provides an upper bound on the time that a route can remain suppressed. The default maximum suppression time is 60 minutes. To modify the default, include the max-suppress option at the [edit policy-options damping name] hierarchy level.

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.

The merit ceiling, e_c, which is the maximum merit that a flapping route can collect, is calculated using the following formula:

ec £ er 2(t/l) (ln 2)

e_r is the figure-of-merit reuse threshold, t is the maximum hold-down time in minutes, and l is the half-life in minutes. For example, if you use the default figure-of-merit values in this formula, but use a half-life of 30 minutes, the calculation is as follows:

ec £ 750 2(60/30) (ln 2) 

ec £ 3000

   
NOTE: The cutoff threshold, which you configure using the suppress option, must be less than or equal to the merit ceiling, ec. If the configured cutoff threshold or the default cutoff threshold is greater than the merit ceiling, the route is never suppressed and damping never occurs. 


 

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 router, use the show route detail command.