Routing protocol faults cause problems ranging from an inability to communicate to excessive routing overhead. This paper proposes a system for detecting a wide range of routing protocol faults. Our system deploys virtual routers called RouteMonitors to monitor a routing protocol. We deployed RouteMonitors in the MBone's DVMRP infrastructure and uncovered a number of faults. We were also able to identify the cause of these errors and deploy fixes. The result was a substantial improvement in DVMRP route stability. This paper reviews the RouteMonitor approach and details the results achieved in the MBone.