Dave, as a red-handed “terminology-abusing marketer”, I’d like to offer up a brief explanation of the term “L3 switching” to help clarify what the term means, where it came from, and why it has become a bit of an anachronism in today’s technology world. It’s an interesting example of how technical terms evolve and change over time.

Networking professionals know that network “switching” (as a technology) means forwarding decisions are made using layer 2 information (the MAC address). The term “routing” means those decisions are made using layer 3 information (the IP address). Way back in the early days of networking (6 or 7 years ago!), “routers” were complex, expensive, and relatively slow products that used software running on general-purpose processors to move traffic. Switches, which made simpler forwarding decisions based on the MAC address, were generally faster and less expensive. As a result, “switches” (the products) rapidly gained popularity in Enterprise networks.

“Layer 3 switching” was born from the insight that once the initial “routing” decision was made, the switch’s MAC address table could be programmed with the source MAC/destination MAC address pair for faster “switched” forwarding of subsequent traffic. So the slower “routing” technology established the initial path and “switching” moved subsequent traffic.

Today, the taxonomy for networking has evolved. We now reserve the word “router” for big products that support intra-domain routing protocols like BGP or for small devices (think SOHO or branch) that support LAN/WAN interfaces and provide network address translation services. We use the term “switch” primarily to describe products that support only Ethernet media and inter-domain routing protocols like OSPF. Nearly all Enterprise-grade switch products can use layer 3 information to make forwarding decisions. So if you hear a vendor refer to an “L3 switch,” that means the product probably has only Ethernet interfaces and it is capable of making forwarding decisions using IP information. An “L2 switch” meets the more classic definition of a switch because it is not typically capable of making forwarding decisions using L3 information. To complicate matters, many advanced L2 switches ARE capable of making access control decisions (not forwarding decisions) using L3 information.

That’s where the divergence in terminology enters the picture. “Switching” and “routing” as technologies are still what we learned in school, but “switches” and “routers” as product categories have evolved – not necessarily due to terminology abuse (well, okay, maybe :)), but due to the fact that “routing” technologies have crept into the “switch” product category.

Scott Lucas
Extreme Networks