Cisco Express Forwarding (CEF)

Cisco Express Forwarding (CEF)
Two of the newer additions to the Cisco range (the 3550 and 4000 series) are sometimes
described as multilayer switches, with the obvious inference that something beyond legacy
routing is going on. With respect to the 3550 series and both the 4000 and 6500 series using
the Supervisor IV engine, that something is Cisco Express Forwarding (CEF). In fact, the
3550 is advertised as supporting CEF-based multilayer switching.
CEF differs from other MLS implementations, in that there is no caching in the traditional
sense. Caching introduces a number of issues that need to be addressed. For example, how long
should a cache stay valid? How big should a cache be permitted to grow? And how do we deal
with routing topology changes that invalidate cache entries?
Well, Cisco has constantly worked to try to optimize cache behavior, but the problems
remain. It seems that the only good way to do layer 3 data forwarding is to use a routing
table. But that slows everything down again, right? Actually no, not necessarily. You see,
if you create a stripped-down version of the routing table and a separate adjacency table
(which is similar to a separate ARP cache), then you can get the best of both worlds. The
table resides close to the interfaces (figuratively speaking), keeping data away from the busy
route processor and its buses. And because the table is in communication with the main
routing table, it is always as up-to-date as the main table.