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How To Configure Cisco Express Forwarding (CEF)

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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:

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

Distributed CEF Mode

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.

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Hall of Fame Super Gold

Not pertinent to this forum.

Also appears outdate (VIP) and poorly titled - "resolution".

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