AT&T via Tata and Level3

Thu Mar 3 20:12:16 UTC 2011

On Thu, Mar 03, 2011 at 11:15:51AM -0500, Morgan Miskell wrote:
> I've noticed that we have thousands of routes for AT&T via Tata that 
> we don't have from AT&T through Level3.  I would expect Level3 to have 
> most of the routes for AT&T that Tata does since they are both 
> directly peered with AT&T.

Well, I don't know anything about this specific issue or any policy 
changes that may have been made, but at a high level I can tell you that 
BGP doesn't work like that. BGP is only capable of passing on a single 
best path for each route, and what is considered the best path is 
totally in the eye of the beholder.

First off you must understand that the vast majority of Internet routes 
are multi-homed at some level. As you get into large Tier 1 carriers, 
the amount of overlap is massive (i.e. you'll hear the same route as a 
"customer" from multiple networks), and the question of which path will 
be selected is completely up to the policies of the network doing the 
selecting. Not only does this vary by policy, but it varies by the 
composition of other networks they peer with (or buy from), what other 
networks buy from them, and even their network topology (due to tie 
breaking rules like EBGP > IBGP).

For example, Level 3 is a much larger network with significantly more 
customer routes than Tata. I'm too lazy to do an actual comparison 
between the two, but odds are high that of the AT&T customer routes that 
they announce to their peers, probably somewhere around 30-40% of those 
routes are also Level 3 customer routes as well. A network will ALWAYS 
prefer their customer routes above those learned from peers (or else 
they wouldn't be able to guarantee that they're actually providing full 
transit service), so those routes coming from AT&T will never be 
selected. Meanwhile, Tata is receiving those same routes from both AT&T 
and Level 3 (and potentially other peers and/or customers too), and is 
completely free to make their own best path selections based on their 
own local criteria.

The result is that you should almost never expect to see the same paths 
for the same networks being selected by two different large networks, 
unless the routes in question are single homed and there are no other 
choices (which is a small minority of the routes on the Internet).

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)