[76710] in North American Network Operators' Group
Re: Dampening considered harmful? (Was: Re: verizon.net and other email grief)
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Tue Dec 21 05:19:28 2004
In-Reply-To: <a06200700bded74efb20d@[66.6.34.245]>
Cc: nanog@merit.edu
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Tue, 21 Dec 2004 11:18:51 +0100
To: Jerry Pasker <info@n-connect.net>
Errors-To: owner-nanog-outgoing@merit.edu
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.
