Some networks (of note, the larger ones) have registered a "customer ASN". The idea is that networks advertised from their backbone ASN should only be the ones they own, and all customers who have no ASN use the customer ASN to originate their block. In most cases the contract prohibits using the customer ASN with another provider; it is only to be used to single home to the one network.
I have no personal experience with AT&T in this configuration, but
with several other networks they would prefer an eBGP session where
they send you a default and you send them your prefix using the ASN
they assign. Aside from keeping the prefixes segregated by ASN it
also makes the routing policy a lot simpler. Typically things
announced by the backbone ASN may appear in prefix lists across the
network, while the customer ASN is "just another session".
One of the more interesting "big network" problems is the front
line support tend to not be creative thinkers, and also tend to
believe their internal terminology is industry standard speak. This
can make it difficult to get what you want.
--
Leo Bicknell - [EMAIL PROTECTED] - CCIE 3440
PGP keys at http://www.ufp.org/~bicknell/
Read TMBG List - [EMAIL PROTECTED], www.tmbg.org
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