North American Network Operators Group
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Re: Dampening considered harmful? (Was: Re: verizon.net and other email grief)
- From: Iljitsch van Beijnum
- Date: Tue Dec 21 05:20:22 2004
On 21-dec-04, at 9:16, Jerry Pasker wrote:
IF there's a connection problem, or implementation difference that
makes a lot of up/down, then dampening could occur close to the
"problem" but it will be contained close, and won't spread to the rest
of the internet.
Today's AS hierarchy is quite flat, which severely limits the
usefulness of dampening. If the link between ASes A and B flaps, then B
doesn't get to dampen these flaps. C, connected to D, does, but if C is
a small network that doesn't help much as flap dampening brings its own
overhead. In a two or three router network there probably isn't any
advantage in dampening. Only when you get to protect a larger number of
routers from the update, it helps. Now of course D, connected to C,
will be isolated from the instability. But in today's internet, there
often isn't a D. According to the weekly routing table report the
current average AS path length is 4.5. Subtract at least .5 for
prepending, and there must be a significant number of 3 or even 2 AS
hop paths to get the average at 4.
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