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Re: Aggressive route flap dampening
From: Tony Li <tli () jnx com>
Date: 07 Jul 1997 22:57:37 -0700


smd () clock org (Sean M. Doran) writes:

The "N" should be reduced (or the time period lengthened) and the 
cost of increasing that ratio should increase with the length of
the prefix, in order to encourage topologically sound aggregation
either through traditional means or through NAT and NAT-like boxes
such as the one described and implemented by Paul Vixie.

Here's where we part company.  Varying the 'charge', either monetary or in
flap penalty creates an incorrect incentive: folks are incented to use a
shorter prefix.  Note that this is distinct from aggregation in that they
may simply use a shorter prefix and not actually use more address space,
thus hurting netwide utilization.  

The 'correct' incentive is a charge for flapping and a charge for
announcement.  Both have real costs, directly traceable to processor and
memory costs.  

It fixed two problems simultaneously: firstly, there is lots of flap
and flap is most irritating when relatively unimportant (and statistically
small is likely to be less important than large) NLRI is responsible for
a disproportionally large amount of it.  Secondly, there are lots of
networks which really ought to be aggregated.  When a single up/down or
up/down/up flap makes the network unusable for an hour or two, people
generally become motivated either to be very very stable or to aggregate
even adjacent aggregatable /24s in order to suffer fewer disconnectivities.

Note that both of these are 'fixed' without a length restriction: the per
prefix charge incents folks to aggregate.  The per flap charge incents them
to stability.  Direct cause and effect, without harmful side effects.  ;-)

Tony


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