The current design of BGP implicitly assumes the existence of trust between ASes with respect to exchanging valid BGP updates. This assumption of complete trust is problematic given the frequent announcement of invalid -- inaccurate or unnecessary -- updates. This paper presents AS-CRED, a reputation service for ASes which quantifies the level of trust one can have with respect to its announcing valid updates. To compute the reputation, AS-CRED analyzes the past updates announced by each observable AS in the Internet, over a time-window, based on well-defined properties. It then classifies the resulting observations into multiple types of feedback. The feedback values are input into a mathematical function for computing AS reputation. The reputation is then used to track the instances of invalid updates announced in the Internet and trigger alerts. The contributions of the paper are: (1) a reputation service for ASes, characterizing their trustworthiness; (2) a set of well-defined properties for analyzing AS behavior; (3) a simple reputation function and feedback mechanism; (4) a reputation portal which regularly publishes AS reputation; and (5) a reputation-based alert service which tracks potentially invalid updates in the Internet. Detailed analysis of AS-CRED demonstrates: (a) AS behavior is repetitive making reputation an effective trust metric, and (b) AS-CRED's alerts for invalid updates show an eight fold improvement over existing alert systems.