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

Mon Mar 28 10:54:54 UTC 2005

--On 27 March 2005 12:59 -0800 Randy Bush <randy at psg.com> wrote:

> better?  i did not say better.  a simple way to look at it, which
> we have repeated here every year since com-priv migrated here is
>
> a tier-1 network does not get transit prefixes from any other
> network and peers with, among others, other tier-1 networks.
>
> a tier-2 gets transit of some form from another network, usually but
> not necessarily a tier-1, and may peer with other networks.
>
> this does not please everyone, especially folk who buy transit and
> don't like discussing it.  and there are kinky corners

Even this is debatable (& I know you know this Randy).

Firstly, peering isn't binary. Is peering vs transit a distinction based on
routes taken / accepted & readvertised, or on cost? Does "paid for peering"
count as peering or transit? If you pay by volume? If you pay for "more
than your fair share" of the interconnect pipes? (if the latter, I am
guessing there are actually no Tier 1s as everyone reckons they pay for
more than their fair share...).

Secondly, it doesn't cover scenarios that have have happened in the past.
For instance, the route swap. EG Imagine networks X1, X2, X3, X4 are "Tier
1" as Randy describes them. Network Y peers with all the above except X1.
Network Z peers with all the above except X2. Y & Z peer. To avoid Y or Z
needing to take transit, Y sends Z X2's routes (and sends Z's routes to X2
routes marked "no export" to X2's peers), and Z sends Y X1's routes (and
sends Y's routes to X1 marked "no export" to X1's peers). Perhaps they do
this for free. Perhaps they charge eachother for it and settle up at the
end of each month. Perhaps it's one company that's just bought another.

All this come down to the fact that "Tier n" is not a useful taxonomy
because there is no clear ordering of networks.

If I was really pushed for a definition, I'd say it was this: you are a
Tier-1 network, when, if you tell all third parties not to advertise your
routes to anyone but their customers, and you get a phone call from one of
your customers complaining about a resultant connectivity problem, you can
be confident before you've analyzed it, that the customer will accept
it's that networks problem, not yours. This boils down to "does the
customer believe you".

Alex