Service Providers behaviour for dual homed enterprises

Wed Sep 23 21:38:02 UTC 2015

I've always worked in enterprise only so I thought you guys might be able
to help me with this one.
We are dual homed to Verizon and AT&T.  We prepend all our prefixes out
AT&T to make them least preferred.  During a recent issue we found some
users were coming in via AT&T.  Using various looking glasses it looks like
if I use an AT&T server(route-server.ip.att.net) the best path is the
prepended route through AT&T; in fact,I don't even see the VZB route.  If I
use a 3rd party looking glass(router-server.he.net) I see what I
anticipated, which is the shorter AS-Path through VZB.

So if my research is correct, the internet prefers Verizon UNLESS they are
a direct AT&T customer then they would use the AT&T circuit.
Is this a standard practice that I should assume to encounter?

Thanks in advance