Cisco's Express Forwarding (CEF) technology for IP is a scalable, distributed, layer 3 switching solution designed to meet the future performance requirements of the Internet and Enterprise networks. It represents the latest advance in Cisco IOSTM switching capabilities that includes NetFlowTM Switching and Distributed Switching. CEF is also a key component of Cisco's Tag Switching architecture.
Express Forwarding evolved to best accommodate the changing network dynamics and traffic characteristics resulting from increasing numbers of short duration flows typically associated with Web-based applications and interactive type sessions. Existing layer 3 switching paradigms use a route-cache model to maintain a fast lookup table for destination network prefixes. The route-cache entries are traffic-driven in that the first packet to a new destination is routed via routing table information and as part of that forwarding operation, a route-cache entry for that destination is then added. This allows subsequent packets flows to that same destination network to be switched based on an efficient route-cache match. These entries are periodically aged out to keep the route cache current and can be immediately invalidated if the network topology changes. This 'demand-caching' scheme — maintaining a very fast access subset of the routing topology information — is optimized for scenarios whereby the majority of traffic flows are associated with a subset of destinations. However, given that traffic profiles at the core of the Internet (and potentially within some large Enterprise networks) are no longer resembling this model, a new switching paradigm was required that would eliminate the increasing cache maintenance resulting from growing numbers of topologically dispersed destinations and dynamic network changes.
CEF avoids the potential overhead of continuous cache churn by instead using a Forwarding Information Base (FIB) for the destination switching decision which mirrors the entire contents of the IP routing table. i.e. there is a one-to-one correspondence between FIB table entries and routing table prefixes; therefore no need to maintain a route-cache.
This offers significant benefits in terms of performance, scalability, network resilience and functionality, particularly in large complex networks with dynamic traffic patterns.
