AT&T routing issue

Charles Gucker cgucker at
Wed Nov 5 02:51:53 UTC 2008

On Tue, Nov 4, 2008 at 6:20 PM, Campbell, Alex
<Alex.Campbell at> wrote:
> Hi all,
> We have recently brought up some BGP sessions with a new provider, and
> have found that we can get end to end connectivity through the new
> provider to pretty much everywhere except networks behind AT&T (AS7018).
> We are able to see our routes in AT&T's route server, but all
> traceroutes from our network to hosts behind 7018 stop at AT&T's border.
> We are multihomed, so I've local-prefed up our secondary ISP and the
> problem resolves immediately.
> Our upstream (AS9443) and their upstream (AS11867) have been chasing
> this up with AT&T for a couple of days, but don't seem to have had made
> much progress.

In short yes.   AT&T uses a customer specific access list to perform a
uRPF like function.   That is, if your provider did not request for
their provider to have AT&T update their filter.

> Does anyone have any suggestions as to what could be causing this, or a
> contact at AT&T who might be able to assist?

Since you are experiencing this issue only when you send your traffic
out (eventually though AT&T) it points to the inbound ACL on AT&T's
border.    I have been bitten by this too many times, especially when
customers, or customers of customers don't properly inform the other
of the new netblock.      I would be curious to find out if the issue
you are facing is specific with every network you are announcing or
just one or two networks.     As for who would assist, this request is
suppose to go through:

AT&T MIS Maintenance
888-613-6330 Prompt-3, 2
rm-awmis at


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