The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

4.4. MultiLayer Switching

Up to now, all of our discussion has concentrated on switching (bridging) at the Data Link layer (Layer 2) of the OSI model. Remember that the term "switch" was simply market-speak for a high-performance implementation of a bridge. When bridge technology was first developed, it was impractical to build wire-speed bridges with large numbers of high-speed ports. With improved silicon technology, we were able to move many functions previously implemented in software into the hardware, thus increasing performance and enabling manufacturers to build reasonably priced wire-speed switches.

A MultiLayer Switch (MLS) is a switch that performs all of the traditional functions at Layer 2, while also functioning at a Layer 3 device and sometimes Layer 4. The MLS will implement Quality of Service (QOS). The MLS is also able to route between ports and/or vlans at wirespeed because the MLS uses hardware instead of software for routing operations. In the next section, we discuss this in more detail.

4.4.1. Layer 3 Switching

Refer back to section 4.4 where we discussed the evolution from a bridge to a switch; a very similar phenomenon occurred in the evolution of Network layer routers. Routers provide functionality beyond that offered by Data Link layer bridges; as a result, they naturally entail greater complexity. Like early bridges, routers were traditionally implemented in software, often running on a special-purpose communications processing platform. For a given ...