For technical, managerial, and sometimes political reasons, the Internet routing system consists of two components - interior routing and exterior routing. The concept of an Autonomous System (AS), as define in Section 2.2.4 of this document, plays a key role in separating interior from an exterior routing, as this concept allows to deliniate the set of routers where a change from interior to exterior routing occurs. An IP datagram may have to traverse the routers of two or more Autonomous Systems to reach its destination, and the Autonomous Systems must provide each other with topology information to allow such forwarding. Interior gateway protocols (IGPs) are used to distribute routing information within an AS (i.e., intra-AS routing). Exterior gateway protocols are used to exchange routing information among ASs (i.e., inter-AS routing).