This paper presents the results from a detailed, experimental study of OSPF, an intra-domain routing protocol, running on a mid-size regional Internet service provider. Using multiple, distributed probes running custom monitoring tools, we collected continuous protocol information for a full year. We use this data to analyze the health of the network including the amount, source, duration and periodicity of routing instability. We found that information from external routing protocols produces significant levels of instability within OSPF. We also examine the evolution of the routing topology over time, showing that short term changes are incremental and that the long term trend shows constant change. Finally, we present a set of detailed investigations into several large scale anomalies. These anomalies demonstrate the significant impact external routing protocols have on OSPF. In addition, they highlight the need for new network management tools that can incorporate information from routing protocols.