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| 5.1 |  | Multilayer Switching |
| 5.1.1 |  | Multilayer switching |

The term multilayer switching (MLS) is used for two distinct things. On one hand, it is a general network design term that refers to hardware-based PDU header rewriting and forwarding, based on information specific to one or more OSI layers.

On the other hand, as characterized in this module, MLS refers to a specific technology employed by various Cisco devices to perform wire-speed PDU header rewrites and forwarding. To help differentiate between the two uses of the term multilayer switching, when the acronym MLS is used (as opposed to the expanded term), it refers to the Cisco specific technology.

MLS was the first technological step in solving the router-as-a-bottleneck issue. It is also responsible for blurring the lines we have taken for granted with respect to using the OSI model as a means of delineating per-OSI-layer functions. With MLS, Layer 3 switching speeds approximate those of Layer 2 switching. This has a dramatic impact on network design. The trend is to incorporate more and more Layer 3 functionality into what are traditionally Layer 2 devices. For example, the Catalyst 3550 access-layer switch, uses CEF by default, making it a true multilayer switch.

The discussion of MLS will begin by detailing the hardware and software requirements followed by an explanation of MLS components and operations.