There was a disagreement a couple of years ago between Cogent and Level 3 (whom are termed two of the internet's largest Tier 1 providers which means that they operate the core backbones of the net)
Level 3 alleged that Cogent was presenting more traffic to its network than it was taking in return which Cogent denied, in response, :Level 3 stopped aanouncing routes to Cogent customers or routing traffic from Cogent's customers to those it peered with or who bought transit from it which lead to huge swathes of the Internet becoming inacessible where an ISP/hosting provider/company did not either have connections to both or to parties not involved in the dispute.
This is one of the reasons Internet Exchanges such as the London InterNet eXchange (LINX) of which PN are a member and the PAIX (Palo Alto Internet Exchange) in the States developed so that providers could share the costs of setting up and running a shared switch infrastructure which they could peer over, thus providing more efficient connections by reducing round trip times and allowing peering sessions to be established with the minimum of hardware requirements while the switch hardware was owned by a not for profit third party which they all owned equally by paying a subscription based on the connection capacity (the minimum for LINX I believe is a 100 Mbps port on a LINX switch), you can find more info on their site at
http://www.linx.netI think networking is a fascinating subject which is why I am studying to move into this area
