An understanding of certain network functions is critical for successful network management. Managers must have insight into network topology, protocol performance and fault detection/isolation. The ability to obtain suchin sight is even more critical when trying to support evolving technologies. Multicast is one example of a new network layer technology and is the focus of this paper. For multicast, the pace of change is rapid, modifications to routing mechanisms are frequent, and faults are common. In this paper we describe a tool, called Mantra, we have developed to monitor multicast. Mantra collects, analyzes, and visualizes network-layer (routing and topology) data about the global multicast infrastructure. The two most important functions of Mantra are: (1) monitoring multicast networks on a global scale; and (2) presenting results in the form of intuitive visualizations.