performance - cef is less cpu intensive than fast switching or route caching, giving way to processing for other layer 3 services such as qos
scalability - cef enables full switching capacity for line cards, however it is also used on fixed configuration switches
resilience - because it uses fib lookups which contain all known route destinations; fast switch processing and route cache dependance are eliminated
the primary components of CEF are the forward information base and adjacency tables
adjacency table maintains layer 2 linked to a particular fib... this avoids arp reliance
types of adjacency
cache adjacency - this entry contains the outbound interface and the corresponding mac address for its fib entry. the mac is the destination subnet that is directly connected, or the mac of the subnet the packet must be forwarded to to reach the destination
receive adjacency - this entry handles packets whose final destination is THIS router. includes packets assigned to the router, broadcast packets, and multicasts which have as their destination THIS router
null adjacency - handles null interface packets; these packets are usually dropped
punt adjacency - packets that cannot be handled by cef, usually forwarded to fast switching layer for forwarding
glean adjacency - created when when the router knows the directly connected destination ip subnet and does NOT know the destination mac, or knows the forwarding router's destination subnet but does NOT know that router's mac address. these packets generate an arp request
discard adjacency - fib entries to these adjacencies are discarded
drop adjacency - packets to this entry are dropped but the prefix is checked
dCEF or distributed cef is recommended when there is a fib on each line card in a chassis. this precludes the main processor and/or route table queries for next hop ip's. fast switching is instead performed
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