consistent policy != consistent announcements

Alex.Bligh amb at
Fri Mar 14 22:22:57 UTC 1997

> 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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