Flapping /16 prefix, why?

Jim Rowh jim at mci.net
Fri Feb 21 13:39:49 UTC 1997

The degree of prepending you show is probably more for Sprint's 
benefit than anyone else's. 1793, 1239, and 1790 are all Sprint AS's. 
If you're lucky, you can find someone familiar with Sprint's 
confederated routing scheme to explain the details. AS3505 
belongs to VNet, who uses both Sprint and Gridnet as providers. Prepending 
allows them (VNet) to effectively designate Gridnet as their primary 
(all things being equal, better--shorter--AS path wins).

As for hold-down routes (to Null0) and damping, yes, they're both 
good and generally practiced ideas. As to whether and to what extent 
Sprint uses either or both of them, you will have to take that up 
with them: noc at sprint.net or 1-800-230-5108


> Sorry for littering NANOG with this email, but I'm hoping for a little
> education on this matter from some of the larger National Service
> Providers.
> This morning we had a customer call in concerning a network hole.
> Upon investigation, I found the following..
> BGP routing table entry for, version 541808
> Paths: (1 available, best #1, advertised over IBGP, EBGP)
>   1793 1239 1790 3505 3505
> from (
>       Origin IGP, metric 0, valid, external, best
>       Community: 1
>       Dampinfo: penalty 656, flapped 18 times in 02:19:29
> Two immediate things came to mind here.  
> 1.	This is a /16 that is flapping.
> 2.	The end user is using AS path prepending for some reason.
> With these two facts in mind, the following questions arise.
> Anchoring or tying down of a route with a static route to null seems
> to be a very basic idea (far more basic that AS path prepending), so 
> why isn't this route tied down?
> I only see one route to this prefix, why are they prepending?  (I admit
> that only the user of this prefix can answer this question, and I also 
> admit that with 53 flaps on the books so far, the other provider may
> be dampening the route for us *thank you*)
> Finally, does Sprint not apply *any* dampening to customer routes?
> My cat is long dead (10 years at least), so it's safe from my curiosity.
> Chris A. Icide
> Nap.Net, L.L.C.

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