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 of routing anomalies and robust to failures.

We then propose a new form of interconnection that overcomes the limitations of route election and route redistribution. We conduct a thorough study of this new form of interconnection, presenting conditions for correctness of the whole routing system and conditions for the optimality of the paths it finds. The precepts are applied to routing instances substantiated by the current Internal Gateway Protocols of the Internet: RIP, OSPF, IS-IS, IGRP, and EIGRP.