Foreword

Tag Switching, the Cisco proprietary technology that evolved into MPLS began in March 1996. At that time, several major ISPs were operating two-tiered networks in order to manage the traffic in their network. You see, IP always takes the shortest path to a destination. This characteristic is important to the scalability of the Internet because it permits routing to be largely an automatic process. However, the shortest path is not always the fastest path or the most lightly loaded. Furthermore, in any non-traffic-engineered network, you find a distribution of link utilizations, with a few links being very heavily loaded and many links being very lightly loaded. You end up with many network users competing for the resources of the busy links, while other links are underutilized. Neither service levels nor operational costs are optimized. In fact, one ISP claims that, with Traffic Engineering, it can offer the same level of service with only 60 percent of the links it would need without Traffic Engineering.

Thus, Traffic Engineering becomes an economic necessity, enough of a necessity to build a whole separate Layer 2 network. To engineer traffic, an ISP would create a mesh of links (virtual circuits) between major sites in its IP network and would use the Layer 2 network, either Frame Relay or ATM, to explicitly route traffic by how they routed these virtual circuits.

By April 1996, it was recognized at Cisco that tag switching offered a means of creating explicit routes within the IP cloud, eliminating the need for a two-tiered network. Because this held the potential for major cost savings to ISPs, work began in earnest shortly thereafter. Detailed requirements and technical approaches were worked out with several ISP and equipment vendors.

Eric Osborne and Ajay Simha work in the development group at Cisco that built Traffic Engineering. They have been actively involved in the deployment of Traffic Engineering in many networks. They are among those with the greatest hands-on experience with this application. This book is the product of their experience. It offers an in-depth, yet practical, explanation of the various elements that make up the Traffic Engineering application: routing, path selection, and signalling. Throughout, these explanations are related back to the actual configuration commands and examples. The result is a book of great interest to anyone curious about Traffic Engineering and an invaluable guide to anyone deploying Traffic Engineering.

George Swallow
Cisco Systems, Inc.
Architect for Traffic Engineering and Co-Chair of the IETF's MPLS Working Group