Abstract. Studies have shown that the Internet has experienced the widespread failures such as router crash, fiber cut, or scheduled maintenance. Ideally, routing protocols should be able to quickly find alternate paths to reroute around failures. In this paper, we consider one important factor that prevents routing protocols from achieving this goal: the delay of finding and obtaining alternate paths, defined as routing lag. We show that a significant number of routing failures in the Internet are caused by routing lags, and they can last for a significant period of time.