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Re: route for linx.net in Level3?
From: Brian Dickson <brian.peter.dickson () gmail com>
Date: Thu, 4 Apr 2013 15:53:39 -0400

Leo Bicknell wrote:

Even if the exchange does not advertise the

exchange LAN, it's probably the case that it is in the IGP (or at

least IBGP) of everyone connected to it, and by extension all of

their customers with a default route pointed at them.

Actually, that may not be the case, and probably *should* not be the case.

Here's why, in a nutshell:

If two regional ISPs on either side of the planet, point default to the
same Global ISP,
even if they do not peer with that ISP, by using the IX next-hop at IX A
(for ISP A),
and IX B (for ISP B), then the Global ISP is now giving free on-net transit
to A and B.

So, it turns out that pretty much the only way to prevent this at a routing
is to not carry IXP networks (in IGP or IBGP), but rather to do

The other way is to filter at a packet level on ingress, based on Layer 2
which on many kinds of IX-capable hardware, is actually impossible.

So, when it comes to IXPs: Next-Hop-Self.

(BCP 38 actually doesn't even enter into it, oddly enough.)


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