Discussion

figure-of-merit value correlates to the probability of a route’s future instability, and the
value decays exponentially over time. BGP suppresses routes with higher figure-ofmerit
values for longer periods of time.
For a new route, BGP assigns a figure-of-merit value of 0. If the route experiences any
instability, the value is increased based on the following rules:
• If the route is withdrawn by the EBGP peer, the value increases by 1,000.
• If the route is readvertised by the EBGP peer, the value increases by 1,000.
• If the BGP attributes for the route change in a new Update message from the
EBGP peer, the value increases by 500.
The points, or demerits, given to a route decrease over time and decay exponentially.
This time is the half-life of the route. If the demerits decay faster than the figure-ofmerit
value increases, the route will not be suppressed. When the figure-of-merit value
increases beyond a cutoff value, called the suppression threshold (also called the cutoff
threshold), the route is suppressed and is considered unusable. The router will ignore
any new information about the route received from its peers and will not install it
into the forwarding table or forward the route to any other routing protocols. The
figure-of-merit value continues to decay based on the half-life. When the value drops
below the reuse threshold, it is unsuppressed and again considered usable.
The damping parameters that you configure play into the figure of merit. The
suppress statement controls the suppression threshold. By default, when a route’s
figure-of-merit value reaches 3,000, it is suppressed. The figure-of-merit value decays
exponentially over the half-life that you set with the half-life statement. The default
half-life is 15 minutes. To illustrate how the decay works, if a route has a figure-ofmerit
value of 1,000 and no incidents occur, the value decays to 500 after 15 minutes,
then to 250 after another 15 minutes. You set the reuse threshold with the
reuse statement. The default is 750. As the figure-of-merit value continues to decay,
when it drops below the reuse threshold, the route becomes usable again. The maximum
amount of time a route can be suppressed is 60 minutes by default, which you
can modify with the max-suppress statement.
The first step in configuring damping parameters is to create a named parameter list.
In this recipe, the list is named damping-normal, which sets up a standard set of
damping parameters. The figure-of-merit value decays over 15 minutes, which is the
default half-life. Routes are suppressed when their figure of merit reaches a value of
6,000 (instead of the default 3,000) and are unsuppressed at half that value (3,000)
instead of at the default value (750). Finally, in the recipe, routes remain suppressed
for a maximum of 30 minutes instead of the default 60 minutes.
The figure of merit doesn’t increase forever but stops when it reaches the merit ceiling,
ε c , which is a value that is calculated based on the reuse threshold (ε r ); half-life
(λ), in minutes; and maximum suppression time (t), in minutes:
ε c ≤ε r ∗ e (t/l) (ln 2)
Mitigating Route Instabilities with Route Flap Damping | 459
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