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Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications, or interactive sessions
Enable CEF when your router has interface processors that do not support dCEF. To enable CEF, use the ip cef command in global configuration mode.
Enable dCEF when you want your line cards to perform express forwarding so that the route processor (RP) can handle routing protocols or switch packets from legacy interface processors. To enable dCEF operation, use the ip cef distributed command in global configuration mode as needed.
Benefits
CEF offers the following benefits:
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Improved performance—CEF is less CPU-intensive than fast switching route caching. More CPU processing power can be dedicated to Layer 3 services such as quality of service (QoS) and encryption.
•Scalability—CEF offers full switching capacity at each line card when distributed CEF (dCEF) mode is active.
•Resilience—CEF offers unprecedented level of switching consistency and stability in large dynamic networks. In dynamic networks, fast switching cache entries are frequently invalidated due to routing changes. These changes can cause traffic to be process switched using the routing table, rather than fast switched using the route cache. Because the Forwarding Information Base (FIB) lookup table contains all known routes that exist in the routing table, it eliminates route cache maintenance and the fast switch/process switch forwarding scenario. CEF can switch traffic more efficiently than typical demand caching schemes.
Restrictions
•The Cisco 12000 series Gigabit Switch Routers operate only in distributedCEF mode.
•Distributed CEF switching cannot be configured on the same VIP card as distributed fast switchin.g
•Distributed CEF is not supported on Cisco 7200 series routers.
•If you enable CEF and then create an access list that uses the log keyword, the packets that match the access list are not CEF switched. They are fast switched. Logging disables CEF.
CEF Operation Modes
CEF can be enabled in one of two modes:
Central CEF Mode
When CEF mode is enabled, the CEF FIB and adjacency tables reside on the route processor, and the route processor performs the express forwarding. You can use CEF mode when line cards are not available for CEF switching or when you need to use features not compatible with distributed CEF switching.
Note On the Cisco 12000 series Internet router, dCEF is enabled by default. The command to enable dCEF is not available. Also, the configuration file does not indicate that dCEF is enabled on the router.
Distributed CEF Mode
When dCEF is enabled, line cards, such as VIP line cards or GSR line cards, maintain an identical copy of the FIB and adjacency tables. The line cards perform the express forwarding between port adapters, relieving the RSP of involvement in the switching operation.
dCEF uses an Inter Process Communication (IPC) mechanism to ensure synchronization of FIBs and adjacency tables on the route processor and line cards
For more detail, see: Configuring Cisco Express Forwarding.
