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Re: Cogent --> Google Public DNS routing issue
From: "Patrick W. Gilmore" <patrick () ianai net>
Date: Wed, 17 Aug 2011 09:13:00 -0400

On Aug 17, 2011, at 1:07 AM, Christopher Morrow wrote:
On Wed, Aug 17, 2011 at 12:09 AM, Robert Glover <robertg () garlic com> wrote:

We have noticed that from our Cogent link (as well as from ALL U.S. based
points we tested via the Cogent Looking Glass:
http://www.cogentco.com/en/network/looking-glass), traceroutes to
and all seem to go over to Europe: ain't the driods you are looking for...

In the traceroute appended to the original post, he did trace to

While it did go all over, I don't see the problem - it got to the destination host.

Anycast is OK for some things, but it depends on BGP.  BGP has zero concept of latency, loss, or geography.  Expecting 
anycast to guarantee an optimal path or location is a grave error.

The possible reasons for this are nearly innumerable.  Perhaps Congent <> Google is congested in the US so one or the 
other prefers EU?  Perhaps there is some IGP metric messed up inside Cogent that prefers the EU?  Perhaps more 
nefarious problems, such as Google de-peering Cogent in the US?  Etc., etc.

You may be able to find out if you look, and you may not (I didn't even try).  But even if you do figure out the 
answer, you can't fix it.  Only Cogent and/or Google can.

Moreover, you can see things like this with anycast even when there is no problem!


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