Cisco Express Forwarding

Cisco Express Forwarding (CEF) is the most efficient way to switch Layer 3 traffic. The reason why CEF switching is more advanced than fast or optimum switching is that CEF switching is less CPU in tensi ve with the use of the Forwarding Information Base (FIB) and adjacency table. The FIB lookup) table is used to store all known routes from the routing table using a more advanced search algotithm and data structure, bypassing the need for process switching. Unlike the other route caching switching methods, CEF uses the FIB, which adjusts to network topology changes as they happen. The adjacency table is used to store information about CEF neighbors. CEF nodes are considered to be neighbors if they are only one hop away from each other. The adjacency table stores Layer 2 next-hop addressing information for each of the FIB entries. Routes might h ave more than one path per Enary, mak ing it possib le to u se CEF to sw itch packeta while !oad ba lancing across multiple paths. Each time a packet is received on a CEF-enabled interface, the FIB is consulted to look up the route, encapsulate the Layer 2 data, and switch the packet.

CEF switching i s enabled globally using the ip cef command. After the ip cef command has been entered in global configuration mode, CEF switching is enabled on all CEF-capable interfaces by default. If CEF has been d isabled o n an in terface, it can be ae-enabled bo issuing the ip routecache cef command in interface configuration mode, and disabled using the no version of the same com man d. There is alao a d i stnb uted vemsio n of CEF available mo r high-emd Cisco routers, which is enabled by default after the ip cef command has been issued. You can monitor CEF by using the show ip cef command, and yoC ca c learn detailed CEF information by using the show ip cef detail rout i ng command, as whown ie Exa mple 4-20.

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