fastswitching

Fast switching is a packet-forwarding technique used in Cisco IOS routers that temporarily caches the result of routing decisions to speed up subsequent packets to the same destination. It is part of the legacy IOS forwarding path, sitting between process switching and Cisco Express Forwarding (CEF).

Operation: When a packet arrives, the router first checks the fast-switching cache for an entry keyed by the destination address. If a cache entry exists, the packet is forwarded using the cached outgoing interface and next hop without consulting the control plane. If there is a cache miss, the router performs a normal routing lookup (process switching for the first packet), forwards the packet, and creates a cache entry so that subsequent packets to that destination can be switched quickly.

Cache details: The fast-switching cache stores per-destination entries with the outbound interface, next hop, and related state needed to forward the packet. Cache entries are invalidated when the topology or routing table changes, or when the cache is flushed due to resource constraints. The mechanism reduces CPU load compared with process switching but still requires software intervention for cache misses.

Relation to other mechanisms: Fast switching provides a performance improvement over process switching but is generally slower and less scalable than Cisco Express Forwarding (CEF). In modern IOS deployments, CEF is the default and preferred forwarding method; fast switching is often available for compatibility but not relied upon for new designs.

History and usage: Fast switching was introduced to improve performance on early IOS platforms and on carriers where CEF was not yet adopted. With modern hardware and software, it is commonly disabled or rarely used, with CEF handling the forwarding plane.