the lateset and prefered Cisco IOS platform switching mechanism is CEF, which incorporates the best of the previous switching mechanisms. CEF supports per-packet load balancing (previously supported only by process switching), per-source or per-destination load balancing, fast destination lookup, and many other features not supported by other switching mechanisms.
The CEF cache, or Forwarding Information Base (FIB) table, is essentially a replacement for the standard routing table.
CEF uses a complete IP switching table, the FIB table, which holds the same information as the IP routing table. The generation of entries in the FIB table is not packet-triggered but change-triggered. When something changes in the IP routing table, the change is also reflected in the FIB table.
Because the FIB contains the complete IP switching table, the router can make definitive decisions based on the information in it. Whenever a router receives a packet that should be CEF-switched, but the destination is not in the FIB, the packet is dropped.
The FIB table is also different from other fast-switching caches in that it does not contain information about the outgoing interface and the corresponding Layer 2 header. That information is stored in a separate table, the adjacency table. This table is more or less a copy of the ARP cache, but instead of holding only the destination MAC address, it holds the Layer 2 header
Cisco Express Forwarding (CEF) is the foundation on which MPLS and its services operate on a Cisco router. Therefore, CEF is a prerequisite to implement MPLS on all Cisco platforms except traditional ATM switches that support only data plane functionality. CEF is a proprietary switching mechanism used on Cisco routers that enhances the simplicity and the IPv4 forwarding performance of a router manifold
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