Layer 3 Switch
If you tell a big enough and keep repeating it, people will eventually come to beve it. --Joseph Goebbels
This is yet one more example why I dislike "marketing hype ".
Note that unlike most of our other pages, this page does not start with a definition. This is because there is no such thing as a Layer 3 switch. Don't buy into the marketing hype and don't pay more because the manufacturer claims the device is a 'layer 3 switch' and is therefore faster than X brand of router.
Cisco's smaller competitors are the ones that started this--I used to remember who actually was the first to claim they did layer 3 switching. My eyes popped out of their head, I laughed for about five minutes and then tossed out the ad when I saw it and forgot about it.
Cisco knows better than to claim they have layer 3 switches, but their competition continues to claim to have 'layer 3 switching'. Because of this mis-information by competitors, Cisco's customers are demanding 'layer 3 switches', even though such a thing does not (and cannot) exist. Many of Cisco's customer's don't know the difference between what routing and switching actually does and how it works. So, Cisco is selling those customers what they are really asking for, which is a layer 2 switch that can do routing from an ASIC. They honestly assess the customer requirements and provide the device that meets those requirements.
This lack of knowledge on the customer's part regarding the technology is part of why Cisco's website contains such a wealth of information on how routing and switching work.
The Marketing SPIN
Here are the false assumptions that marketing departments spread via dis-information and mis-information campaigns and literature:
- There is no difference between a switching and routing and no difference between switches and routers.
- Switching is always better than routing
- The difference between switching and routing is the same as the difference between software and hardware.
- Brand X device is a hardware-based Layer 3 switch, therefore it's faster than Brand Y.
All of these claims are dubious, at best and outright untruths at their worst.
Where the Error LIES
Let's tackle each of these issues one at a time:
There is no difference between a switching and routing and no difference between switches and routers.
Once upon a time, the hardware necessary to perform switching would only fit in its own device. The same was true with routers. As every generation of integrated chips gets smaller, faster and more powerful, it becomes possible to fit more and more functionality inside a single device. The more functions that are squashed into a single device, the more blurred the lines get between the switch and router in most people's eyes.
Marketing hype often states, or imps that because the device has both switching and routing capabilities that switching and routing are really is the same thing. This is often very subtle as they transpose the terms frame and packet when talking about switching capabilities; the literature contains phrases such as 'switches packets' and 'packet switching'.
But, just because you can fit router chips in the same device as switch chips doesn't make it a layer 3 switch. Even when the switching and routing are done in a magical, proprietary application-specific integrated chip (ASIC), the output of the device still results in two separate functions: packet routing and frame switching.
Let me clarify what it is that routers and switches do:
Layer 2 (Switches)
- Layer 2 deals with Frames
- Switches (in general) forward Frames to a destination end station
- Switches don't change the IP header of the Frames they forward
- Switches send layer 2 broadcasts
Layer 3 (Routers)
- Layer 3 deals with packets
- Routers forward Packets to a destination network
- Routers change the IP Header of the Packets they forward
- Routers block layer 2 broadcasts
To summarize, routing and switching operate at completely different networking layers. They are NOT the same thing and they do NOT perform the same functions, therefore there is no device that can perform layer 3 switching.
Please note that nobody claims to do layer 2 routing, which is just as silly.
Switching is always better than routing
This is not true. Routers break up layer 2 broadcast domains. This reduces the overall total of frames on the network and reduces the bandwidth used by the network. Switches create and send layer 2 broadcasts out all interfaces particpating in the LAN, thereby creating the congestion. If your layer 2 broadcast domain is large enough, you won't have very much throughput. Routers are necessary, even if that routing functionality is an add-on inside the switch.
The difference between switching and routing is the same as the difference between software and hardware.
Again, False. Although routing functions are now performed in hardware, the output of the device is still a routed packet, not a switched frame. Moving the functionality for routing from the software to hardware may speed up throughput on the device, but it doesn't change the fact that a modified packet is being forwarded, not an unmodified frame.
Brand X device is a hardware-based Layer 3 switch, therefore it's faster than Brand Y.
Cisco has ASIC's in their devices. They did, in fact, pioneer this technology in networking gear, so it's nothing new to Cisco. Once their competition started doing it, and then started with the marketing 'layer 3 switching' hype, Cisco had to keep up. Any company can build an ASIC. But even ASIC's still must output data that other devices which conform to networking standards can recognize, so the end result is still routed packets and switched frames.
Caveat
Please bear in mind that this article is addressed to those who are interested in purchasing a Layer 3 switch and I have erred on the side of readability and clarity rather than technical accuracy.
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