Cisco Express Forwarding (CEF) is the newest, and fastest, switching method available in IOS. CEF was invented to help overcome the major deficiencies of the fast switching method. Reviewing the description of fast switching for a moment, the major deficiencies noted were:
Lack of support for overlapping cache entries.
Any change in the routing table or ARP cache results in the invalidation of large sections of the route cache, because there is little or no correlation between the route cache and the tables on which the cache is based.
The first packet to any given destination must be process switched to build a route cache entry.
Inefficient load balancing in some situations (primarily in cases where many hosts are transacting ...