Route Policies
7705 SAR OS Router Configuration Guide 415
The Alcatel-Lucent implementation of route damping is based on the following parameters:
• Figure of Merit — a route is assigned a Figure of Merit (FoM), which is proportional
to the frequency of flaps. The FoM algorithm can characterize a route’s behavior over
a period of time. See Damping for more information on FoM and damping.
• route flap — a route flap is not limited to the withdrawn route. It also applies to any
change in the AS path or the next hop of a reachable route. A change in AS path or
next hop indicates that the intermediate AS or the route-advertising peer is not
suppressing flapping routes at the source or during the propagation. Even if the route
is accepted as a stable route, the data packets destined for the route could experience
unstable routing due to the unstable AS path or next hop.
• suppress threshold — when the configured suppress threshold is exceeded, the route
is suppressed and not advertised to other peers. The state of the route is considered
to be down from the perspective of the routing protocol.
• reuse threshold — when the FoM value falls below the configured reuse threshold
and the route is still reachable, the route is advertised to other peers. The FoM value
decays exponentially after a route is suppressed.
The two events that could trigger the route flapping algorithm are:
• route flapping — if a route flap is detected within a configured maximum route flap
history time, the route’s FoM is initialized and the route is marked as a potentially
unstable route. Every time a route flaps, the FoM is increased and the route is
suppressed if the FoM crosses the suppress threshold.
• route reuse timer trigger — a suppressed route’s FoM decays exponentially. When it
crosses the reuse threshold, the route is eligible for advertisement if it is still
reachable.
If the route continues to flap, the FoM, with respect to time scale, looks like a sawtooth
waveform with the exponential rise and decay of FoM. To control flapping, the following
parameters can be configured:
• half-life — the half-life value is the time, expressed in minutes, required for a route
to remain stable in order for one half of the FoM value to be reduced. For example,
if the half-life value is 6 (min) and the route remains stable for 6 min, then the new
FoM value is 3. After another 6 min passes and the route remains stable, the new FoM
value is 1.5.
• max-suppress — the maximum suppression time, expressed in minutes, is the
maximum amount of time that a route can remain suppressed
• suppress — if the FoM value exceeds the configured integer value, the route is
suppressed for use or inclusion in advertisements
• reuse — if the FoM value falls below the configured reuse value, then the route can
be reused
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