This MIB module defines management objects for Cisco Layer 2/3
switches. These devices may either have a single (central) switching
engine entity or may consist of multiple (distributed) switching
engine entities which are inter-connected via a common 'switching
fabric'. In the central switching engine model, all the physical
ports in the system are handled by the only switching engine in the
system.  In the distributed switching model, each switching engine
will handle a set of 'local' physical ports and when necessary,
packets are also switched between switching engines over the
switching fabric.

Cisco L2/L3 switching devices use regular routers to assist them
in learning packet 'flows' by observing how a router routes a
candidate flow. A flow is some combination of source network address,
destination network address and the transport port numbers, as
applicable.  Once a flow is established (learned), all traffic
belonging to that flow will be switched at Layer 3 by the switch
engine, effectively bypassing the router, until the flow has been
'aged' out. Most Cisco L2/L3 switching devices employ built-in
(internal) router module(s) for integrating Layer 3 switching with
Layer 2 forwarding. However, they can also learn 'flows' through
other physically-separate (external) Cisco routers that are
connected to the switch-engine through the network.