Flapping /16 prefix, why?

Rob Liebschutz rob at rjl.com
Fri Feb 21 19:03:54 UTC 1997

> 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?

If their is a link flapping somewhere between the border of the AS
where the route is announced and your network, your going to see flapping
regardless of weather the route is tied down.

I previously though that tying down my routes was the best thing.
This may be so for the heath of the Internet, but I've had substantial
problems with the LAN interface on one of my routers.  Everytime the
LAN interface goes down, my routes would get blackholed.  So the
Health of the Internet may be better if I tie down my route, but, my
customers will be happier if a failure on my LAN interface causes my
traffic to be routed over another NAP instead of blackholed.  On
another of my routers, on which I've never had problems with the LAN
interface, I do tie the route 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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