Routing and switching
Routing and switching is a somewhat unfortunate, but widely accepted term in computer and telecommunications networking. With respect to discrete units of data (i.e., packets and frames), routing at the internetwork layer is the dominant technology, supplemented by bridging and hybrid technologies such as Multi-Protocol Label Switching (MPLS). Circuit switching is declining in use, but has different techniques for redirecting physical or virtual information streams. Asynchronous transfer mode (ATM), also called cell switching, was a now-obsolescent method that tried to merge the benefits of routing and bridging; MPLS has been called "ATM without cells".
Bridging, MPLS, and circuit switching do not primarily operate at the internetwork layer, but below it, although they may take information from it. When the Internet Engineering Task Force was dealing with MPLS and some other things that "don't quite fit", and some people insisted on calling it "layer 2.5", the reality is that the IETF set up a "Sub-IP Area" and did the original work there. MPLS is now back under the Routing Area. There was also a Performance Implications of Link Characteristics (PILC) working group that has ended its effort, but also deals with sub-IP
In the general networking marketplace, "switching" is more of a marketing term than a formal definition. In the 1990s, when routers were relatively slow and essentially software-only implementations on general-purpose computers, one major vendor coined the phrase, "switch when you can, route when you must." Underlying that slogan was the idea that bridges took advantage both of specialized hardware and a lack of understanding of fast routing software, so bridging was faster than routing. That difference, however, has largely disappeared. Still, marketeers began to refer to routers as "layer 3 switches" or other terms implying they were fast. Indeed, some specialized routers were differentiated on specific hardware acceleration techniques.
- http://www.isi.edu/pilc/ PILC archives