What is the fundamental difference between an L3 switch and a router?

Question

Computer networks

What is the fundamental difference between an L3 switch and a router?

Answer 1

In terms of purpose and internal structure)
L3-switch "drives" (passes without significant change) traffic well, does normal forwarding between networks, and can dynamically route. From the initial packet, it basically changes the source and destination MAC addresses, QoS and CRC. Sure, fancy switches can do GRE, some even IPSec, but you won't get much performance out of it.
The router just "threshes" data packets - actively changes headers, encapsulates in tunnels, natit, terminates ppp networks. He, as you can see, actively changes the contents of the packets, climbing deeper than the ethernet level.
Hence the difference in the internal device - inside the switch ASICs and QoS processors, inside the router - smart processors and various other gadgets for their own purposes (hardware encryption support). the amount of missed traffic, and for the router - in the number of processed packets.
Well, yes, the difference, of course, is in the set of functions) It is clear that the same isis in most cases can do both the switch and the router. But if you compare by feature navigator, for example, you will see that in fact, switches can do less.

Answer 2

In my opinion, an L3 switch has less L3 functionality than a router, but, as a rule, its device is optimized to maximize performance. Roughly speaking, an L3 switch can do fewer different things, but it does them better.
The router, as a rule, is able to raise VPN tunnels, sometimes (less and less often with the spread of Ethernet everywhere) routers support L2 technologies other than Ethernet. Often, routers have rich security functionality (built-in firewall, ips, etc.).
The L3 switch is able to exchange routes with other devices and redirect (“forward”) traffic. In order to increase performance, specialized hardware solutions are used (ASIC for traffic redirection, hardware support for prefix trees, etc.). Since hardware resources are expensive, according to some numerical characteristics (the number of routes, for example), the L3 switch may lag behind the router.
In addition, the L3 switch supports switching technologies (xSTP, etherchannel, etc.)
Thus, the router has rich L3 functionality, the L3 switch has narrower, but optimized, L3 functionality and, in addition, functionality " conventional switch.
Finally, one should not forget that all such abstractions and categorizations are rather arbitrary and one can find counterexamples to them in real life.

Answer 3

Router - routes.
Switch - switches :D
In other words, the router is able to build routes between different subnets , operate dynamic / static routing protocols using the third level of the OSI model.
The switch, originally a second-level device, is able to use the third level only in a few cases, for example, in building VLANs.
The switch does not accept ip-routes, cannot serve as a gateway, and, in principle, is intended for other purposes.
A simple analogy, for example: Bicycle and Motorcycle. The point is about the same. The purpose, and hence the method of application, are completely different.

Answer 4

As an addendum to @throughtheether's answer , I 'll add.
In one of the Cisco books, the following explanation was given approximately: Cisco invented the router, therefore it advises to use routing rather than switching, i.e. at the distribution level, create local VLANs and use end-to-end VLANs (which extend throughout the network) as little as possible. And in order for there to be connectivity between local VLANs, and in order to be able to get out of them, for example, to the Internet, routing is necessary. That's what they just added to the distribution switches - so they became L3 switches. Gradually, they began to acquire more and more functionality, and the line between the router and the L3 switch became smaller and smaller.
Therefore, we can conditionally say that L3 switches provide routing within the company, and at the border there is already a full-fledged router that provides access to the external network.