I would suggest that the largest percentage of flapping prefixes in the
global routing system belong to prefixes longer than /19.
Hence the convention to damp differently for different lengths. See one of
the foils in http://www.psg.com/~randy/970210.nanog/, which suggests that we
over here start following the European lead on this.
Is the route computation of a /8 prefix flapping once a second any
different than a /24 flapping once a second? If /8's are "naturally"
more stable, then why allow them to flap more before dampening them?
When dampening was first being rolled out I remember one of the early
networks that got hit was PSI's net 38/8. Treating flapping prefixes
differently based on length has more to do with how many people scream
when prefixes covering a large amount of address space get dampened
than the impact of the route flap of an individual prefix on the router.
Although most folks have permanently filtered it, isn't 1/8 still the
flappiest prefix of all.
When dampening was first being rolled out I remember one of the early
networks that got hit was PSI's net 38/8. Treating flapping prefixes
differently based on length has more to do with how many people scream
when prefixes covering a large amount of address space get dampened
than the impact of the route flap of an individual prefix on the router.
Also, it is thought that longer prefixes tend to flap more than shorter.
randy
Also, it is thought that longer prefixes tend to flap more than shorter.
That's not because of the prefix length per se; it's because shorter
prefixes tend to be associated with a greater number of reachable
destinations per prefix, and that tends to imply better infrastructure
and more opportunities for aggregation and hold-ups.
--apb (Alan Barrett)
It's just another incentive to renumber into larger aggregate blocks. A
provider can say to a customer:
"If your /24 flaps you're going to be unreachable from some parts of the
net for longer than you would if you renumber into our block."
pbd