AS prepending

Sat Apr 9 20:33:37 UTC 2005

On Fri, 8 Apr 2005, Patrick W Gilmore wrote:
>
> On Apr 8, 2005, at 10:28 AM, Philip Lavine wrote:
>
> > I am using AS prepending to favor one ISP over
> > another, in a BGP multihomed/multiISP scenario. Why
> > does the ISP receiving the prepends fail to add my
> > network into their routing table? Is this a "feature"
> > of BGP, or have I gone too far with 3 prepend
> > statements.
>
> If they are both transit providers, then they are broken.

I have found at least one major ISP did check the amount of prepends and
did not accept the announcement if it did not match exactly what was coded
in their as-path filter.  They had to code special as-path filters that
allowed an undetermined number of prepends so we could "play".

Why not just ask your upstream ISP if they filter prepends?

-Hank
>
> If they are peers, the second ISP is probably preferring the route it
> hears through your transit provider because there are fewer AS hops.