Information Technology Reference
In-Depth Information
Prefix Update Optimization
Prefix update optimization is focused on preventing instabilities in prefix advertisement
and minimizing the impact of new policy application. This involves detecting instabilities
external to the network and filtering the instabilities to the internal network. Another aspect
is minimizing the impact of making policy changes and reducing the amount of prefix
information advertised. The following features cover the available prefix-handling
optimizations:
Route flap dampening —This feature monitors routing information for signs of
instability. Prefixes that demonstrate instability are dampened until they stabilize.
Soft reconfiguration and route refresh —The soft reconfiguration and route refresh
features are designed to minimize the impact of applying new policy through reducing
the impact on unaffected prefixes.
Transmit (TX) side loop detection —Transmit side loop detection is focused on
reducing prefix information sent to an external peer. If the prefix information will be
rejected by the remote peer because of the AS_PATH loop-detection mechanism, the
update process can be optimized by suppressing the advertisement of those prefixes.
Outbound Route Filtering (ORF) —The ORF capability is similar in concept to the
TX side loop detection in that it focuses on reducing the prefix information advertised
by a peer based on inbound policy configuration on the remote peer. The ORF feature
specifically focuses on offloading the inbound prefix filtering on the transmitting peer.
Route Flap Dampening
The route flap dampening feature is described in RFC 2439. It has three major goals:
Provide a mechanism to reduce router processing load caused by unstable routes.
Prevent sustained route oscillations.
Provide increased route stability without sacrificing route convergence time for
generally well-behaved routes.
Route dampening maintains a route flap history for each prefix. The dampening algorithm
has several parameters:
History state —After a single route flap, the route is assigned a penalty, and the
dampening state for the route is set to History.
Penalty —Each time the route flaps, the penalty increases. The default penalty increase
for a route flap is 1000. If the route attributes are the only change, the penalty increase is
500. This value is hard-coded.
Suppress limit —If the penalty exceeds the suppress limit, the route is dampened. The
route state is changed from History to Damp. The default suppress limit is 2000.
The suppress limit can be configured.
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