Date: Thu, 3 Mar 2011 14:12:16 -0600 From: Richard A Steenbergen <ras@e-gerbil.net> Subject: Re: AT&T via Tata and Level3 Cc: nanog@nanog.org
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.
[[.. sneck much good stuff ..]] While what you say is accurate, it is _irrelevant_ to the situation that the OP posted about. Methinks you misunderstood what he said. He peers with Level3 and TATA. Both of whom peer with AT&T. Looking at the -incoming- data from those two peers, he sees "thousands" of entries for AT&T address-blocks announced to him by TATA that are not being announced to him by Level3. Postulating that AT&T _is_ announcing all its address-blocks to both of those direct peers, the 'one-BGP-hop-removed-from-directly-connected' network should expect to see all those blocks from any of it's directly connected peers that are directly connected to AT&T. If one of those peers sees a 'better' route to one of those AT&T address-blocks, then it should be announcing that indirect path instead of the direct one. Ditto for blocks that AT&D does -not- announce (for whatever reason, traffic engineering, maybe?) to a directly connected peer. I would hazard a guess that the "missing routes" _might_ be the result of supressing 'more specifics', or they _are_ being announced to Level3, but with a 'community' tag that Level3 interprets as 'use locally, but do not announce externally'.