Cisco Express Forwarding (CEF) is a Layer 3 (L3) IP switching technology. CEF is faster, less CPU-intensive and provides superior performance compared to other switching technologies, especially in networks with dynamic traffic patterns. It supports per-source-destination and per-packet load balancing. This is a major advantage over route-cache switching mechanisms like fast switching, where route caching needs to be disabled for per-packet load balancing to work. CEF support in Cisco IOS is version and platform dependent.
To confirm that CEF is enabled globally, issue the show ip cef command from the user EXEC or privileged EXEC mode. The show ip cef command displays the entries in the Forwarding Information Base (FIB). If CEF is not running, the %CEF not running message appears instead. To display CEF information for all interfaces, issue the show cef interface command from the user EXEC or privileged EXEC mode.
Specify the interface type and number to view CEF information for a specific interface only. At times, interfaces configured for CEF switching might not be able to CEF switch a packet due to a missing adjacency entry. This is due to a feature not supported by CEF or a feature that requires special handling. Such packets are switched using the next-best switching mode. To verify the switching path being used to switch packets, issue the show interface stats command from the user EXEC or privileged EXEC mode. To determine if the route cache counter indicates fast-switched or CEF-switched packets, issue the show ip cache command and look for the counter called IP routing cache entries. If packets are being CEF-switched, no entries are displayed in the fast-switched cache.