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Re: Disaster recovery using as-prepend?
From: Todd Vierling <tv () duh org>
Date: Fri, 17 Feb 2006 16:57:12 -0500 (Eastern Standard Time)

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 () duh org> <tv () pobox com> <todd () vierling name>

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