Since large-scale Internetworks such as the public Internet must continue to support escalating growth due to increasing demand, it is clear that Autonomous Systems (ASes) must avoid injecting excessive de-aggregated prefixes into the interdomain routing system and instead mitigate de-aggregation internally. This document describes an Interior Routing Overlay Network (IRON) architecture that supports sustainable growth within AS-interior routing domains while requiring no changes to end systems and no changes to the exterior routing system. In addition to routing scaling, IRON further addresses other important issues including mobility management, mobile networks, multihoming, traffic engineering, NAT traversal and security. While business considerations are an important determining factor for widespread adoption, they are out of scope for this document.