In Grid environments, high-performance applications have to take into account the available network performance between the individual sites. Existing monitoring tools like the Network Weather Service (NWS) measure bandwidth and latency of end-to-end network paths. This information is necessary but not sufficient. With more than two participating sites, simultaneous transmissions may collide with each other on shared links of the wide-area network. If this occurs, applications may obtain lower network performance than predicted by NWS. In this paper, we describe Topo Mon, a monitoring tool for Grid networks that augments NWS with additional sensors for the routes between the sites of a Grid environment. Our tool conforms to the Grid Monitoring Architecture (GMA) denned by the Global Grid Forum. It unites NWS performance and topology discovery in a single monitoring architecture. Our topology consumer process collects route information between the sites of a Grid environment and derives the overall topology for utilization by application programs and communication libraries. The topology can also be visualized for Grid application developers.