Figure 17-13 Process Switching with the CPU
Involved in Packet Routing Decisions
Fast switching: Fast switching is the next-level evolution of process
switching. The CPU is involved for only the first packet that is routed. It
determines the outgoing interface required for the packet and updates
the route cache, which typically resides in the kernel. Once the route
cache is populated, the input interface driver does a route-cache lookup
and fast switches the packet to the outgoing interface. Figure 17-14
shows fast switching, in which the processor essentially updates the
route cache after it learns the next hop. All subsequent packets are fast
switched.Figure 17-14 Fast Switching and the Route Cache
Cisco Express Forwarding (CEF) switching: CEF switching is an
advanced Cisco-proprietary form of fast switching. CEF switching
sometimes involves specialized hardware and distributed routing, and
the CPU may not need to get involved in the data path at all. It does this
by building a route cache with forwarding information; this is known as