Multimedia applications often involve one-to-many, or many-to-many, communication. These applications can be supported efficiently with network layer multicast. While multicast is a promising technology, monitoring multicast routing infrastructure and the performance of applications that use it presents challenges not found with unicast. In this paper we describe a monitoring architecture, and an implementation of the architecture, that uses application level monitoring to assess the performance of multicast-capable infrastructure. The architecture makes use of standards-based management technologies and allows both active and passive monitoring. The implementation has been tested extensively and provides a network operator with a network-wide view of multicast performance.