This paper presents the architecture of a passive monitoring system installed within the Sprint IP backbone network. This system differs from other packet monitoring systems in that it collects packet-level traces from multiple links within the network and provides the capability to correlate the data using highly accurate GPS timestamps. After a thorough description of the monitoring systems, we demonstrate the system's capabilities and the diversity of the results include workload characterization, packet dize analysis, and packet delay incurred through a single backbone router. We conclude with lessons learned from the development of the monitoring infrastructure nd present future research goals.