Legacy Routing and Layer 3 Switching

Legacy Routing and Layer 3 Switching
Over the years, as Cisco routers have matured from the early days of the IGS and AGS platforms,
faster processors have been employed to make the forwarding decisions more quickly.
Nonetheless, it is not only the processor power that determines the latency of a switch. Right
up there with processing delay is the time taken to forward packets around inside the router,
hence the move toward ever faster router architectures.
Designers soon realized that even with faster buses, there were still some delays associated
with internal packet forwarding that might benefit from other techniques, and this gave rise to
the different switching modes employable in modern routers. Because the 3550 and Supervisor
IV–equipped 4000 are really routers as well as switches, these processes suddenly became relevant
to those engineers studying switches.
In order to really see the progression here from legacy routing to layer 3 switching, let’s look
at some of the history, specifically that of process switching (which you could easily call legacy
routing), fast switching, and optimum switching (both cache-based methods for speeding up the
forwarding process). Finally, we’ll look properly at CEF.