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What is FIB (Forwarding Information Base)?

CEF (Cisco Enhanced Forward) uses a Forwarding Information Base (FIB) to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table information is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.

Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and laminates the need for route cache maintenance that is associated with switching paths such as fast switching and optimum switching.

Reference: http://www.cisco.com/en/US/docs/ios/switch/configuration/guide/xcdecef.html