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In-Reply-To: <200412280216.iBS2GO7C002043@noc.mainstreet.net> Date: Tue, 28 Dec 2004 11:33:21 -0600 To: nanog@merit.edu From: Jerry Pasker <info@n-connect.net> Errors-To: owner-nanog-outgoing@merit.edu >Back in mid-December someone typed: > >> > One reason to be careful with dampening is that flaps can be >> > multiplied. (Connect to routeviews and see the different flap counts >> > under different peers for the same flap at your end to observe this.) > >How about in this scenario: > > asA gets transit from asT > asA gets backup transit (ASpath padding) from asB > asB gets transit from asT > asB gets transit from asJ > asJ gets transit from asT > asT peers with whole world(*) > >Now, as asA flaps to asT, we see "bad things" happen to their routes, >namely an unreasonable amount of flap at even nearest neighbors to asT. > >Can this flap magnification be explained by the hierarchy I describe >above? That is, asT treats all of these ASpaths as customer routes: > > asA > asB_asA > asJ_asB_asA > >and so we might expect to see multiple flaps as different "best" >routes come into view inside the geographically diverse asT... right? > >Thanks, >-mark > >(*)you know what I mean. I post this response to the list also seeking clarification/further education. The PATHS that flap at each router get damped, right? So if a path to an AS doesn't flap (best path or not), then it doesn't get damped. So if a best path gets damped, suddenly it's no longer "best" and another non flapping path to that AS becomes the new best. I had to draw this hypothetical network out on a white board, and then it became a little easier to "see". If the path from A to T flaps into oblivion, the path from A to B starts carrying traffic from A<-->B and B<---->J> and J<---->A (via B). The A to T link can flap all day long, and B and J will still have undamped connectivity to A. T will send flaps to J and B and J and B should damp those paths that are flapping, and continue to send traffic to A via the links that aren't flapping. The PATHS that flap at each router get damped, right? If that's the case, then eventually, everyone will damp all the "A" flap from T, and start to prefer any paths to A from J and B (because they aren't flapping, and therefore are better, more stable, and more preferred paths) If T is the only transit provider that's peered with everyone* then A might have a problem. Perhaps A should chose their transit providers a little more carefully. If A, B, and J are all tier 2 ISPs peered with each other that get transit ultimately from T, then none of them are really truly multi homed. If J was also 'nearly peered with everyone' then eventually, A traffic would all start to arrive via J through the connection to B. If all the above is correct, then dampening just made the network much more stable in the above case, rather than traffic constantly routing to and them re routing around the flapping link from A to T. Stability good. Packet loss and latency, bad. Stability = damp. -Jerry
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