Cisco routers switch packets through one of several paths; the characteristics of the switching path depend on whether a route cache is used, how the cache is accessed or built, and what context (process or interrupt) in which the switching actually takes place:
Process switching doesn't cache any information, and switches packets in the context of a process.
Fast switching caches reachability information and the MAC headers needed to forward packets in a radix tree and switches packets during interrupts.
Optimum switching caches reachability and the MAC headers in a multiway tree (mtree), and switches packets during interrupts.
CEF switching caches reachability information in an mtrie, and the MAC headers needed to forward packets within ...