S everal years ago, there were only a handful of deployments of Multi-Protocol Label Switching (MPLS) technology, and those were restricted to researchers or nonproduction networks. Even in practice, MPLS was largely still theory. Over the past several years, MPLS has matured as a technology, and its acceptance and popularity in the marketplace has grown by leaps and bounds. Today a large percentage of the Internet's traffic traverses MPLS-enabled networks run by service providers, and this number seems to only continue to grow. Many of the largest service providers are now using MPLS technology within their networks not only to carry basic core network traffic, but also for the advanced applications and services that can be deployed by using MPLS. For example, there has been a large push by providers recently to deploy virtual private network (VPN) services, both so-called layer-2 and layer-3 VPN services. The demand for this has been driven by customers seeing the benefits and cost savings afforded by using this technology. A sampling of the providers that now deploy MPLS in much or all of their networks includes ATT, British Telecom, UUNet, NTT, France Telecom, and Global Crossing, among others.