[Dshield] Cogent / Level 3

Brian Dessent brian at dessent.net
Fri Oct 7 16:34:18 GMT 2005

John Hardin wrote:

> >From the discussion I've seen (i.e. L3 and Cogent customers *cannot
> communicate at all*) it sounds like L3 is dropping traffic from Cogent's
> netblocks at their boundaries.

No, they're not dropping or blocking the traffic.  There is simply no
route from A to B.

> I mean, simply de-peering shouldn't totally terminate all connectivity
> between L3 customers and Cogent customers, should it? One of the great
> appeals of this intarwebnet thingy is that it routes around outages,
> right? L3 *can't* be Cogent's *only* upstream provider, right? :)

Incorrect.  In theory, BGP would find another path.  In reality, it is
much more complicated than that due to business policies.

The main factor at issue is the notion of peering.  When two autonomous
systems peer, they agree to trade traffic for free.  This is often
called settlement free interconnect because neither party is obligated
to continue the relationship -- it is meant to be a mutually beneficial
agreement that saves both sides money.  Note that the only traffic
traded here is that which is sourced in one peer's AS (or that peer's
downstream) and destined for the other AS (or that peer's downstream.) 
In other words the peers only advertise their own routes.

These peering arrangements are often heavily dictated by business,
politics, and technology.  ISP A might only peer with ISP B if it makes
sense: if both ISPs have nearly equal amounts of traffic for each
other.  If one side has much more traffic for the other, then it's no
longer a mutually beneficial setup.

In contrast to peering is buying transit.  This is where you pay someone
to give you a full view of the internet.  In exchange for money, he
agrees to take any packets you send -- he advertises the entire global
BGP routing table.

Finally there is the notion of a "tier 1" organization.  This term has
been so overused that it is nearly worthless, at least from a marketing
standpoint.  From a technical standpoint, one definition of it might be
a company that is "transit free", or in other words, all their traffic
is handled through peering and they don't have to pay anyone to carry
their packets.  In order to achieve this, the organization must operate
a huge global network and peer with many others.  Because it is so large
and so well connected, it needn't pay anyone for transit because it can
route packets to any destination through one of its many peering
relationships.  This condition is a bit of a chicken and egg however: In
order to be this large the organization must have numerous peering
agreements.  But in order to get those peering agreements, the
organization must be large enough that it is very desirable to others to
peer with them.

Level3 is by most people's definition a tier 1 transit free
organization.  I don't know if they actually do buy transit or not --
they might buy partial transit for some minor routes that are not
covered, but they most certainly do not have to buy full transit. 
Cogent is trying to be a tier 1 organization.  They do buy transit, but
only to reach certain ASes.  They too have many peering relationships.

So herein lies the problem.  Level3 determined that for whatever reason
its peering arrangement with Cogent was no longer mutually beneficial
and terminated the link.  Because neither organization buys full
transit, there is no route between them.  That this will only affect
customers of one that are single-homed that are trying to reach the
other.  Multihomed customers are not affected.

The result of this will either be that one side gives in and
re-establishes the peering link, or one side buys transit to carry its
packets to the other.  It is somewhat of a showdown to see whose
customers complain the most.


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