Coz we haven't had enough arguments like this lately
https://www.reddit.com/r/networking/comments/3wjms6/when_peering_with_bgp_should_the_links_between_as/ (https://www.reddit.com/r/networking/comments/3wjms6/when_peering_with_bgp_should_the_links_between_as/)
takeaway: do not argue with Brian McGahan (or any other 4xCCIE for that matter), even if he is contradicting Internet Routing Architectures!
I hadn't looked at it like that. That's brilliant, thanks! I always reasoned that an external prefix should not be in an IGP if it's not necessary but this is a clear use case.
Very interesting, that's the kind of stuff the books don't teach you! Thanks for posting :)