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