Layer 3 (L3) Switching

Tra-di-tion, Tra-di-tion! Can’t you just hear the song from “Fiddler on the Roof”? Well, tradition is the foundation that you build today upon. That unfortunately means changes in the tradition itself. But before changing a tradition, carefully consider all your options. If you tweak that handed-down pumpkin pie recipe too much, you’ll hear from Uncle Jack or Cousin Gertrude next Thanksgiving.

LAN switches have been L2 devices from the beginning. Modern L3 switches provide higher OSI-level functionalities and can replace routers in the LAN switched environment. Your choice of pure L2 or L3 switches in the enterprise network is no small matter and requires a complete understanding of your network topology and customer needs.

The simple function of an L3 device is to separate network segments. Traditionally, the L3 device was a router. Today, network designers prefer an L3 switch. These two devices differ in the type of information they process to determine the output interface. L2 switching forwards frames based on Data Link layer information (MAC address), while L3 switching forwards frames based on Network layer information (IP address).

A single- or two-L2 broadcast domain backbone isn’t a great solution for large enterprise networks. A better choice for the campus backbone is L3 switches due to scalability and flexibility issues. L3-switched campus backbones are preferred over the L2 backbone for many reasons, including:

• Large scalability potential

• Reduced number of connections between L3 switches. Each L3 distribution switch (router) connects to one L3 campus backbone switch. This simplifies any-to-any connectivity between distribution and backbone switches.

• Flexible topology with no spanning-tree loops. There is no L2 switching either in the backbone or on the distribution links to the backbone because all the links are routed. Arbitrary topologies are supported because of the routing protocol used in the backbone.

• Better support for network services because of L3 support in the backbone switches.

• Multicast and broadcast control. If you have a Layer 2 backbone, you have to create multiple VLANs to provide any type of broadcast control.

One of the many things to consider for the L3 backbone switches is the performance of L3 switching. L3 switches require more sophisticated devices for high-speed packet routing. L3 switches support routing in the hardware, although the hardware in your network may not support all its features. If a certain feature is not supported in the hardware, it must be performed in software, which may impinge on data transfer. Other issues surround QoS and access list support. Hardware cannot process QoS tables or security tables if they consist of too many entries. These limitations will degrade the L3 switch’s performance. And better performance was your goal for using the L3 switch in the first place! If you’re experiencing excessive broadcasts on your corporate LANs, the best solution is to add L3 switching where appropriate and create VLANs.

In addition, an L3 switch uses a route processor, where an L2 switch uses an ASIC chip to make forward/filter decisions.