Many operators run more than one routing instance - more than one routing protocol, or more than one instance of a given routing protocol - in their networks. Route election and route redistribution are mechanisms introduced by router vendors to interconnect routing instances. We show that these mechanisms do not heed basic performance goals. Especially, we show that, in general, they do not allow network configurations that are simultaneously free from routing anomalies and resilient to failures. We then propose a new form of interconnection that overcomes the limitations of route election and route redistribution, permitting the configuration of a resilient and efficient routing system. We conduct a thorough study of this new form of interconnection, presenting conditions for its correctness and optimality. The precepts of the study are applied to routing instances substantiated by the current Internal Gateway Protocols of the Internet: RIP, OSPF, IS-IS, IGRP, and EIGRP. © 2013 IEEE.