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.
This seems to have started around midnight last night. I have a ticket
open with Level3 to inquire...anyone else notice this or anything
similar around 12-1AM EST this morning?
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).
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> 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'.