Stephen Satchell wrote on 9/24/2015 8:39 AM:
On 09/23/2015 02:38 PM, Jason Bullen wrote:
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
That's been my experience, and with other sets of providers, too.
My current company is dual-homed with AT&T and Charter Fiber. Those
customers on UVerse come in the AT&T link no matter what we do with
BGP to convince the cloud to let packets come in the fatter pipe.
Jason, while others have offered acknowledgement of the behavior you are
seeing as well as solutions, I think it might be relevant to point out
that this is simply a matter of BGP best path selection. BGP does not
use AS path length (hops) as its primary path selector. Search for "bgp
best path selection" to find out more about how BGP selects the best
path. As others have noted, local pref is often utilized to control
routing and should be your preferred way to control path selection in
addition to AS path length. However, the ultimate way to control routing
would be to advertise more specific prefixes via the path that you want
traffic to flow.
--Blake
