Service Providers behaviour for dual homed enterprises

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( 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( 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


Many transit providers support BGP communities to modify how your
announced routes are treated within their network. A quick search shows
that AT&T supports BGP community 7018:70 to lower the default local-pref
100 down to 70 (below peer routes). If you tag your AT&T announced
routes with BGP community 7018:70, then even AT&T customers should
prefer to enter via Verizon.


ISP's will generally set a higher LOCAL_PREF toward their customers than
to any other destination out of their network.

It's the money shot.


Hi Jason,

That's normal. Verizon does it too. Both have "community" tags which
you can attach to your route advertisement. Each will have one that
indicates they should give external routes the same "local pref" as
the route you announce to them. Tagging your route announcement with
the proper community will cause them to route based on AS path length
as you expect.

Welcome to the little gotchas of using BGP.

Bill Herrin

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.

Depending on the provider, this can't always be guaranteed, i.e., that
the available LOCAL_PREF values a customer can trigger via a BGP
community support anything <= what routes the network considers "external".

What's possible (or available) may also be influenced by whether one's
upstream is "transit-free" or not, I imagine.


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.


Thank you all for answering. I was disregarding Local Pref because the
route server I was on was showing 100. That was an error on my part though
as it clearly states in the login banner that it is eBGP peering with the
AT&T routers hence the local Pref would go back to 100 from its
perspective. Again, thanks for the quick and thorough responses.

What Blake just said below works best - I do this MED together with
small-ers all the way to india for video conferencing customers sitting in
silicon valley.

Thank You
Bob Evans

Tried that, no joy.

I could only assume then that your peers were either not accepting your advertisements or there was an error in your configuration. All routers will choose the most specific route they have when performing destination based routing. This overrides how the route was installed (static, connected, dynamic) or any metrics considered within each routing protocol for its best path selection.