Cisco Express Forwarding

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Cisco Express Forwarding (CEF) is an advanced layer 3 switching technology used mainly on the Enterprise Core network or the Internet.

[edit] Function

CEF is mainly used to increase packet switching speed reducing the overhead and delays introduced by other routing protocols and increasing the performance.

There are two key components to CEF:

Forwarding Information Base (FIB).
Adjacencies.

The FIB is similar to the routing table generated by multiple routing protocols only maintaining next-hop address for a particular IP-route.
The adjacency maintains layer 2 or switching information linked to a particular FIB entry avoiding the need of an ARP request for each table lookup.
Five types of adjacencies exist:

     * Null adjacency: Handles packets destined to a NULL interface, packets with FIB entries
       pointing to NULL adjacencies will normally be dropped.
     * Punt adjacency: Deals with packets that require special handling or can not be switched by CEF,
       such packets are forwarded to the next switching layer (generally fast switching) so they can be
       forwarded correctly.
     * Glean adjacency: Handles packets destined for currently attached hosts but without layer 2 information.
     * Discard adjacency: FIB entries pointing to this type of adjacency will be discarded.
     * Drop adjacency: Packets pointing to this entry are dropped, but the prefix will be checked.

In order to take full advantage of CEF it is recommended to run distributed CEF(dCEF) where there is a FIB table on each of the linecards. This avoids the need of querying the main processor or routing table in order to get the next-hop information performing the fast switching on the linecard itself.

CEF currently supports Ethernet, Frame Relay, ATM, PPP, FDDI, Tunnels and HDLC.