Now obviously in the real world interconnection between regions within
an ISP network is denser than between ISP networks within regions, but
regional interconnection is still dense enough to make it worth the
trouble.
You are touching on one of the areas that makes me think twice about
geo-aggregation and that is intra-operator routing and policy. It's not
insolvable, and I will not make the claim that I understand this fully
(on the contrary, I am just a hang-around..:) ), but it makes some of
the night-mare set-ups I have seen so far look even worse. Then again,
there is a lot out there that we need to clean up anyway....