Cisco Express Forwarding

Normally a book review shouldn't start with a warning, well this one does: This book is not for everyone. There's a huge number of Cisco books available, many of them dealing with the same technology (routing, switching), some more in-depth than others, and some dedicated to a specific technology. Some people don't know that many packets in a router are switched from an input interface to an output interface. This means that the main CPU of a router is not directly, or is less, involved in the forwarding of a packet. Initially all packets traversing a router were process switched, this had some serious performance issues. So Cisco came up with the idea to cache information to the interface processors. This was the birth of fast switching. Somewhere in the 90's Cisco realized that Fast Switching had its limitations, and a new switching technology was developed which led to CEF (Cisco Express Forwarding). This has become the default switching method in almost all Cisco routers. This book deals with this exclusively. The book has two parts, one dealing with understanding, configuring and troubleshooting (4 chapters), and the second part (3 chapters) has some CEF case studies. Chapter 1 deals mainly with the architecture of a router and has some very detailed information about how memory, buffers and interfaces relate to each other. Chapters 2 and 3 deal with understanding of and enhancements to the original CEF implementations. These two chapters have many show commands to clarify CEF. Chapter 4 has an IP connectivity troubleshooting example in which CEF can help you to understand the problem, an excellent chapter. Chapter 5 describes CEF on a Cat6500, which is hardware based, and the differences when troubleshooting CEF on a Cat6500. Chapter 6 is all about load sharing with CEF. This, for me, is the best chapter of this book. It gives you real world configurations and problems and shows how CEF plays a role in load sharing. Excellent! Chapter 7 deals with CEF in an MPLS VPN environment. Together with chapter 6 it provides really useful information; information you can apply directly in your network. In the beginning I mentioned that this book is not for everyone - let me clarify that. Most people know how to drive a car: use the key to start the engine and off you go, sometimes you have to fill it up. For most people this is enough. Then there are people who understand some of the lights on the dashboard and how to take action on these. But only a few people really understand how a car works, and are capable of dealing with any mechanical problem that might occur. In the same manner, this book will provide the insight required to understand how CEF truly works inside Cisco's routers and switches. This book can promote you to the elite; it is the last piece of the puzzle that will tell exactly how packets are moved inside a router.