Using Routing Policies to Damp BGP Route Flapping | Junos OS (2024)

Configuring BGP Flap Damping Parameters

To define damping parameters, include the damping statement:

The name identifies the group of damping parameters. It cancontain letters, numbers, and hyphens (-) and can be up to 255characters.To include spaces in the name, enclose the entire name in quotationmarks (“”).

You can specify one or more of the dampingparameters described in Table 1.

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 tounderstand how damping suppresses routes. How long a route can besuppressed is based on a figure of merit, whichis a value that correlates to the probability of future instabilityof a route. Routes with higher figure-of-merit values are suppressedfor longer periods of time. The figure-of-merit value decays exponentiallyover 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A figure-of-merit value of zero is assigned to each new route.The value is increased each time the route is withdrawn or readvertised,or when one of its path attributes changes.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 ofinstability, 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• Route is withdrawn—1000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  Other vendors’ implementations for figure-of-merit increasethe value only when a route is withdrawn. The JunosOS implementationfor figure-of-merit increases the value for both route withdrawaland route readvertisem*nt. To accommodate other implementations forfigure-of-merit, multiply the reuse and suppress threshold values by2.

When a route’s figure-of-merit value reaches a particularlevel, called the cutoff or suppressionthreshold, the route is suppressed. If a route is suppressed,the routing table no longer installs the route into the forwardingtable and no longer exports this route to any of the routing protocols.By default, a route is suppressed when its figure-of-merit value reaches3000.To modify this default, include the suppress option atthe [edit policy-options damping name] hierarchy level.

If a route has flapped, but then becomes stable so that noneof the incidents listed previously occur within a configurable amountof time, the figure-of-merit value for the route decays exponentially.The default half-life is 15minutes. For example, for a routewith a figure-of-merit value of1500, if no incidents occur,its figure-of-merit value is reduced to750 after 15minutesand to375 after another 15minutes. To modify the defaulthalf-life, include the half-life option at the [editpolicy-options damping name] hierarchylevel.

A suppressed route becomes reusable when its figure-of-meritvalue decays to a value below a reuse threshold, thus allowing routes that experience transient instability to onceagain be considered valid. The default reuse threshold is750.When the figure-of-merit value passes below the reuse threshold, theroute once again is considered usable and can be installed in theforwarding table and exported from the routing table. To modify thedefault reuse threshold, include the reuse option at the [edit policy-options damping name] hierarchylevel.

The maximum suppression time provides an upper bound on thetime that a route can remain suppressed. The default maximum suppressiontime is 60minutes. 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 whenit reaches a maximum suppression threshold, which is determined basedon the route’s suppression threshold level, half-life, reusethreshold, and maximum hold-down time.

The merit ceiling, ε_c, which is the maximum merit thata flapping route can collect, is calculated using the following formula:

ε_c ≤ ε_r e^{(t/λ) (ln2)}

ε_r is the figure-of-merit reuse threshold, t is the maximumhold-down time in minutes, and λ is the half-life in minutes. For example, ifyou use the default figure-of-merit values in this formula, but usea half-life of 30minutes, the calculation is as follows:

ε_c ≤ 750 e^{(120/30) (ln2)}

ε_c ≤ 12000

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

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

Specifying BGP Flap Damping as the Action in Routing PolicyTerms

To BGP flap damping as the action in a routing policy term,include the damping statement and the name of the configureddamping parameters either as an option of the route-filter statement at the [edit policy-options policy-statement policy-name term term-name from] hierarchy level:

or at the [edit policy-options policy-statement policy-name term term-name then] hierarchy level:

Disabling Damping for Specific Address Prefixes

Normally, you enable or disable damping on a per-peer basis.However, you can disable damping for a specific prefix received froma peer by including the disable option:

Configuring BGP Flap Damping

Enable BGP flap damping and configure damping parameters:

To display damping parameters forthis configuration, use the show policy damping command: