consistent policy != consistent announcements

Rusty Zickefoose rusty at
Sat Mar 15 00:37:01 UTC 1997

	I haven't seen all of the email on this subject, so forgive me if this was all ready mentioned or if it "goes with out saying" but one faily common case would be when a multi-homed customer prefers on provider as primary and the other/s as a backup.  The primary provider would prefer customer announced routes over that of any peers, while the backup providers would set there preferences to normal customers at highest, then peers, and then the customer designated backup at lowest priority.


> > IMO, ISPs who are engaging in the 'hot potato' routing practice have the 
> > obligation to announce consistent routes to its peers at different peering
> > points.  It may require an ISP to change it's internal policy to achive 
> > this with reasonable maintance work (like the one described below) or to
> > excercise whatever internal policy it decide but do more work (configuration
> > management,etc) to fulfill the obligation.
> Peers get announced customer routes. In broad brush-strokes, if you
> don't consistently prefer customer routes over peer routes, and don't
> reannounce peer routes, you are bound to announce routes inconsistently.
> If you do reannounce peer routes, you get into all sorts of possible
> messiness (see below). This much is presumably obvious.
> Is there any *customer-led* reason why one might not want to prefer customer
> routes over peer routes? (i.e. not "it saves me doing some backhaul as
> I can dump the traffic off to the customers other provider").
> If we and X (my peer) both transit Y, I would complain if X didn't prefer
> customer routes over my (peer) routes. Internally-to-X generated traffic
> (news being an obvious example) or other X-customer traffic which gets into
> X's network near a peering point far from where X provides transit to Y,
> I get to do backhaul for. Not nice, not only from a traffic point of
> view but also a debugging one. Also makes asymmetry worse.
> I'd also complain if X started announcing routes like <us> <Y>. Mainly
> from bitter experience where incompetents announced routes like that
> then blackholed the traffic.
> So I'd be interested to know why the customer might prefer anything other
> than "prefer customer routes over peer routes".
> Alex Bligh
> Xara Networks

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