Using Routing Policies to Damp BGP Route Flapping

Configuring BGP Flap Damping Parameters

To define damping parameters, include the damping statement:

[edit policy-options]
damping name {
    disable;
    half-life minutes;
    max-suppress minutes;
    reuse number;
    suppress number;
}

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 (“ ”).

You can specify one or more of the damping parameters 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 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 The value is increased each time the route is withdrawn or readvertised, or when one of its path attributes changes.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• Route is withdrawn—1000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  Other vendors’ implementations for figure-of-merit increase the value only when a route is withdrawn.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. 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 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.

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

ε_c ≤ ε_r e^{(t/λ) (ln 2)}

ε_r is the figure-of-merit reuse threshold, t is the maximum hold-down time in minutes, and λ 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:

ε_c ≤ 750 e^{(120/30) (ln 2)}

ε_c ≤ 12000

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.

Specifying BGP Flap Damping as the Action in Routing Policy Terms

To BGP flap damping as the action in a routing policy term, include the damping statement and the name of the configured damping 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:

[edit policy-options policy-statement policy-name term term-name from]
route-filter destination-prefix match-type {
    damping damping-parameters;
}

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

[edit policy-options policy-statement policy-name term term-name then]
damping damping-parameters;

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 from a peer by including the disable option:

[edit policy-options damping name]
disable;

[edit]
policy-options {
    policy-statement Policy-A {
        from {
            route-filter 10.0.0.0/8 exact;
        }
        then damping none;
    }
    damping none {
        disable;
    }
}

Configuring BGP Flap Damping

Enable BGP flap damping and configure damping parameters:

[edit]
routing-options {
    autonomous-system 666;
}
protocols {
    bgp {
        damping;
        group group1 {
            traceoptions {
                file bgp-log size 1m files 10;
                flag damping;
            }
            import damp;
            type external;
            peer-as 10458;
            neighbor 192.168.2.30;
        }
    }
}
policy-options {
    policy-statement damp {
        from {
            route-filter 192.168.0.0/32 exact {
                damping high;
                accept;
            }
            route-filter 172.16.0.0/32 exact {
                damping medium;
                accept;
            }
            route-filter 10.0.0.0/8 exact {
                damping none;
                accept;
            }
        }
    }
    damping high {
        half-life 30;
        suppress 3000;
        reuse 750;
        max-suppress 60;
    }
    damping medium {
        half-life 15;
        suppress 3000;
        reuse 750;
        max-suppress 45;
    }
    damping none {
        disable;
    }
}

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

user@host> show policy damping
Damping information for "high":
  Halflife: 30 minutes
  Reuse merit: 750 Suppress/cutoff merit: 3000
  Maximum suppress time: 60 minutes
  Computed values:
    Merit ceiling: 3008
    Maximum decay: 24933
Damping information for "medium":
  Halflife: 15 minutes
  Reuse merit: 750 Suppress/cutoff merit: 3000
  Maximum suppress time: 45 minutes
  Computed values:
    Merit ceiling: 6024
    Maximum decay: 12449 
Damping information for "none":
Damping disabled