T1 vs. T2 [WAS: Apology: [Tier-2 reachability and multihoming]]

John Dupuy jdupuy-list at socket.net
Tue Mar 29 20:27:56 UTC 2005

I guess I'm looking at this too much from the point of view of a BGP Admin.

Yes, if you are looking at this from the point of view of payment, then the 
top ISPs do not pay each other.

I was looking at it from a route announcement point of view. Transit is 
where AS A advertises full routes to AS B. Thus, AS B is getting transit 
from A. Peering is where A & B only advertise their network and, possibly, 
the networks that stub or purchase transit from them.

It is my understanding that the top ISPs "trade transit". They provide full 
routes to each other without payment, regardless of how or where the route 
was learned from. They are willing to pass some traffic without 
compensation because it makes for better connectivity. From an announcement 
POV they are not peering.

I am still curious: do any of the larger ISPs on this list want to 
confirm/deny the previous paragraph?

I think we are getting into "defining terms" territory. So, I will bow out 
of the discussion.


At 01:56 PM 3/29/2005, David Barak wrote:

>--- John Dupuy <jdupuy-list at socket.net> wrote:
> > But by the technical description of a "transit free
> > zone", then 701 is not
> > tier one, since I have encountered scenarios where
> > many AS are transversed
> > between 701 and other networks, not just a peer of a
> > peer. Unless, by
> > "transit free zone" you mean "transit trading" where
> > large providers permit
> > each other to transit for free. (Which gets back to
> > my 'who hurts more'
> > discussion.)
> >
>Transit = being someone's customer
>Peering = permitting your customers to go to your
>peer's customers or the peer's network, but not the
>peer's peers, without exchange of money.
>Any other relationship != peering for my purposes
>(although lots of subtly different relationships
>exist, the largest networks tend to take a view which
>is not too dissimilar to the one shown above)
>Are you implying that 701 is paying someone to carry
>their prefixes?  While I'm not the peering coordinator
>for 701, I would find that improbable.  I would expect
>that money would flow the other direction (and thus
>701 would become a more valuable peer for other
> > I'm willing to be wrong. If any of the large
> > providers on the list will say
> > that their network does not transit beyond the
> > customer of a peer; and they
> > still maintain full connectivity, I will gladly be
> > corrected.
>oodles and oodles of people can say this (and already
>have).  A paying customer of mine can readvertise
>(with a non-munged AS_PATH) any of my prefixes which
>they want, and thus provide transit for other people
>to reach me.  That does not change the fact that I'm
>not paying for transit.
>So in short, I would say that T1 vs T2 etc is a
>"follow the money":
>T1 => doesn't pay anyone else to carry their prefixes,
>and runs a default-free network.
>T2 => pays one or more T1 providers to carry their
>prefixes, may or may not run a default-free network.
>T3 => leaf node, pays one or more T1/T2 providers to
>carry their traffic, probably uses default route.
>YMMV, blah blah blah
>David Barak
>Need Geek Rock?  Try The Franchise:
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