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.