Fast Forwarding in Routers

Even though this book is about routing protocols and how to troubleshoot routing- related problems, we would like to briefly mention in this introductory chapter that the high-speed forwarding requirements in today's networks have led to ingenious ways of packet processing on routers that extend beyond basic decision-making based on the IP routing table. The routing table remains critical for routing guidance, but instead of using the contents of the routing table directly, routers transform the routing information in the routing table for storage in data structures, optimized for high-speed packet forwarding. Cisco provides various high-speed forwarding mechanisms, such as fast switching, optimum switching, and Cisco Express Forwarding (CEF).

Frequently, troubleshooting routing problems requires investigation into the fast-forwarding tables, such as the CEF Forwarding Information Base (FIB) and the Adjacency Database. Detailed discussions of these fast-forwarding mechanisms are outside the scope of this book. More information on this subject matter is available at the Cisco site, www.cisco.com.