CEF

yiminggong

Express Forwarding evolved to best accommodate the
changing network dynamics and traffic characteristics resulting from
increasing numbers of short duration flows typically associated with
Web-based applications and interactive type sessions. Existing layer 3
switching paradigms use a route-cache model to maintain a fast lookup
table for destination network prefixes. The route-cache entries are
traffic-driven in that the first packet to a new destination is routed
via routing table information and as part of that forwarding operation,
a route-cache entry for that destination is then added. This allows
subsequent packets flows to that same destination network to be
switched based on an efficient route-cache match. These entries are
periodically aged out to keep the route cache current and can be
immediately invalidated if the network topology changes. This
'demand-caching' scheme — maintaining a very fast access subset of the
routing topology information — is optimized for scenarios whereby the
majority of traffic flows are associated with a subset of destinations.
However, given that traffic profiles at the core of the Internet (and
potentially within some large Enterprise networks) are no longer
resembling this model, a new switching paradigm was required that would
eliminate the increasing cache maintenance resulting from growing
numbers of topologically dispersed destinations and dynamic network
changes.
CEF avoids the potential overhead of continuous cache churn by
instead using a Forwarding Information Base (FIB) for the destination
switching decision which mirrors the entire contents of the IP routing
table. i.e. there is a one-to-one correspondence between FIB table
entries and routing table prefixes; therefore no need to maintain a
route-cache.


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