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I've always read, the router is a layer 3 device, the switch is a layer 2 device, the hub and repeater are layer 1 devices and so on with other devices.

But what does this mean? When I started studying this topic, about 2 months ago, I thought I had understood it, and had defined it as

A layer N device is a device that implements up to layer N of the OSI model and whose main function occurs in that layer N.

So I liked that definition, because I made sense of it.

For example, for a router, it implemented layers 1, 2 and 3 and its main function occurred at the network layer.

However, some time later, I asked myself the question:

If the router implements layers 1,2 and 3 this means that it has to perform all the functions that are executed in layer 1, in layer 2 (framing, error correction/detection, MAC, etc.) and also the functions Layer 3 (Routing, subnetting, etc.)

On the other hand, the Switch implements layer 1 and 2.

So my inference was that the Router would perform all the functions that the Switch does and more, because it implements more layers.

So why would we want a switch, hub or repeater when we have a router that implements the same layers and more?

I know that in reality this is not the case, but it is difficult for me to see what concepts I am confusing and where my definitions are failing.

Thank you very much in advance.

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  • Where does it forward datagrams? Layer-1 forwards bits at layer-1 from one end of a link to another. Bridges (switches are bridges) forward layer-2 datagrams (frames) in the layer-2 domain. Routers forward layer-3 datagrams (packets) between networks. Routers do not forward frames, the strip off the frame to forward the packets and build a new frame for the next network.
    – Ron Maupin
    Jun 10 at 22:38
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The choice of device depends on what is needed for the topology.

Also, as a practical matter, routing functions add significantly to the cost of the device.

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Implementing layers and providing certain functions for layers are different things.

A router forwards at (based on information from) L3. It also uses L2 and L1, necessarily. Although some routers can forward at L2 as well they often do so differently from a switch.

A switch forwards at (based on information from) L2. It also uses L1, necessarily. A simple switch has no concept of layers 3+.

A layer-3 switch can additionally forward at L3. It's still also a layer-2 switch. The main difference from a router is the often limited feature set (no NAT especially) and that a layer-3 switch generally forwards in hardware.

A multi-layer switch may provide various functions for upper layers, blurring the distinction from a (hardware) router almost entirely.

A NIC is also a layer-2 device but it doesn't forward anything, even if it's got multiple ports.

A repeater (hub) is a simple layer-1 device. It forwards bits/symbols from any port to all the others. It has no idea what happens at layers 2+.

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