Software errors and vulnerabilities in core Internet routers have led to several high-profile attacks on the Internet infrastructure and numerous outages. Building an understanding of bugs in open-source router software is a first step towards addressing these problems. In this paper, we study router bugs found in two widely-used open-source router implementations. We evaluate the root cause of bugs, ease of diagnosis and detectability, ease of prevention and avoidance, and their effect on network behavior. Categories and Subject Descriptors: C.2.1 [ComputerCommunication Networks]: Network Architecture and Design General Terms: Reliability