When packets arrive at a Cisco router, the decision about how to forward them depends on the switching path in use. Early in IOS development, routers relied primarily on process switching, where each packet was handled directly by the main CPU. This method was flexible but slow, limiting throughput.
Fast switching was introduced to address performance concerns. The first packet of a flow is process-switched, but the forwarding information is cached. Subsequent packets are switched using this cache, significantly reducing CPU load and improving speed.
As networks evolved, Cisco introduced Cisco Express Forwarding (CEF), which became the default switching method. CEF pre-populates the FIB (Forwarding Information Base) and adjacency tables, allowing for near-instant lookup and forwarding. This approach not only improves performance but also scales better in modern networks.
Understanding when and how these switching paths are applied is vital for troubleshooting high CPU usage or asymmetric routing behavior. It's common to disable CEF on interfaces for testing or troubleshooting, but in production environments, it should remain enabled for consistency and performance.
To inspect switching statistics, use show interfaces switching or show ip cef on Cisco devices. These commands provide insight into how traffic is handled and help validate network design decisions.