Disaster recovery using as-prepend?

Todd Vierling tv at duh.org
Fri Feb 17 21:57:12 UTC 2006

On Fri, 17 Feb 2006, Christopher L. Morrow wrote:

> > > If your primary is connected to ISP_A and the backup is connected to ISP_B,
> > > customers connected to ISP_B MAY still flow to your backup DC (ISP_B will
> > > probably set local preference on all customer routes - you should be able to
> > > override this behavior with communities but not all providers support this (or
> > > honor it 100% of the time!))
> >
> > And in addition to that, even multihomed customers of ISP_B may choose the
> > prepended route for a number of different reasons; for instance, ISP_B might
> > be a cheaper pipe for them, or there may be a smart-ish routing device or
> > scheme in play that overrides normal BGP decision making.
> 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.

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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