Core Issue
Cisco Express Forwarding (CEF) is a Layer 3 (L3) IP switching technology. It is faster and less CPU-intensive than fast switching, and the performance improvement is significant, especially in networks with dynamic traffic patterns. CEF optimizes network performance through optimized lookup for packet forwarding, using special data structures called Forwarding Information Base (FIB) and adjacency tables. The FIB table mirrors the information contained in the IP routing table and eliminates the need for creation and maintenance of the route cache. The adjacency table contains a precomputed link-layer header for the next-hop devices. These devices are used for encapsulating packets. Therefore, these two data structures make packet switching faster.
Resolution
Enable CEF on a router by issuing the ip cef command from global configuration mode. This command enables CEF globally and on all interfaces that support CEF. Optionally, use the distributed keyword to enable distributed CEF on platforms that support it.
To selectively enable or disable CEF on particular interfaces, issue the ip route-cache cef command from interface configuration mode.
For more information on how to configure CEF and resolve related problems, refer to these documents:
