Disaster recovery using as-prepend?

Todd Vierling tv at duh.org
Fri Feb 17 22:07:59 UTC 2006

On Fri, 17 Feb 2006, Todd Vierling wrote:

> > I might be crazy, but couldn't you just prepend the route enough to
> > effectively poison it at ingress to 'backup-isp' ?
> Some route decision override schemes don't care what the path length is at
> all, or factor it in with such a low weight, such that no reasonable amount
> of prepending will change the situation.

And while I actually misunderstood what you said, this is still partly
correct -- ISP_B will often put a preference onto their own route at a level
that outweighs path length absolutely.

If ISP_B has a "don't prefer" community, that could work for ISP_B's own
customers, but short of removing the advertisement from ISP_B's peers too,
that doesn't always move traffic completely off of the ISP_B pipe.  This is
because traffic entering ISP_B destined for the network *will* usually
prefer ISP_B's link in spite of the community, thanks to ISP_B not desiring
to transit packets in and out of their network without a revenue point
somewhere in the path.

> With the development of source traffic engineering schemes, prepending is no
> longer reliable as a means of affecting routing on the remote side.  That
> perception will have to die with it (hopefully sooner rather than later).

-- Todd Vierling <tv at duh.org> <tv at pobox.com> <todd at vierling.name>

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