Switching - fast switching
Fast switching is a switching methodology used by routers and Layer 3 switches where the first packet in a flow is examined by the CPU; the forwarding decision is cached in hardware for the next packets in the same flow.
When packets are fast switched, the first packet is copied to packet memory and the destination network or host is found in the fast-switching cache. The frame is rewritten and sent to the outgoing interface that services the destination. Subsequent packets for the same destination use the same switching path. The interface processor computes the CRC.
all incoming packets are examined by the main CPU and all forwarding decisions are made in software. This is a very slow method, and is typically used by much older Layer 3 devices.