Alas, as anyone who has ever watched Internap when they go flappy flappy can attest, BGP does not handle an excessive number of transit paths very well. I'd really hate to picture the size of the boom that would happen if people WERE to exchange transit paths with each other on anything other than a rare and isolated basis.
True. And I fully support the common practice of heavy filtering on both ends of most BGP sessions to prevent route leakage. Nothing upsets an upstream more than announcing a major network via a smaller connection. Perhaps things have changed a lot in the last six years, which is the last time I got much face-to-face time with other BGP admins. Back then it seemed that the larger networks horse-traded transit pretty regularly. I do not know if was partly automated or case-by-case for each route. (And I suspect it was not always with corporate knowledge.) Especially since some networks (foreign government networks, etc.) were not as "flexible" as one would hope about peering. Again, I'd be interested in hearing from one of the bigger ones on this: UUNet, AT&T, Sprint, Level3, QWest.... If you can't say anything, I understand. John