A growing variety of edge network access devices appear on the marketplace that perform various functions which are meant to complement generic routers? capabilities, such as firewalling, intrusion detection, virus scanning, network address translation, traffic shaping and route optimization. Because these edge network access devices are deployed on the critical path between a user site and its Internet service provider, high availability is crucial to their design. This paper describes the design, construction and evaluation of a general implementation framework for supporting fault tolerance on edge network devices. This implementation framework, called Duplex, is designed to be independent of the functionality of the hosting edge network access device, such that only a minimal amount of programming is required to tailor this framework to a specific edge network access device implementation. Duplex can tolerate power failure, hardware failure, and software failure by supporting device mirroring and watchdog timer-based link bypassing. Empirical performance measurements of an instance of Duplex that is embedded in a commercial bandwidth management device show that the run-time overhead of its fault tolerance mechanisms is less than 1 msec 90% of the time, and the failure detection and recovery period is less than 1.3 sec when running at 100 Mbps.