Doubt in understanding the application of OSI Model

Atharva Bet

Hello,
I hope everyone's safe and healthy.
I had a doubt when it comes to the OSI Model.
The OSI Model essentially is a well-documented set of layers that help in understanding and describing the movement of information across networks right?
Assuming the layers are set in order, I didn't find the same correlation when studying actual networks.
Let's say a number of hosts are connected to a switch that is further connected to a router. Now according to the way these are set in a network, switching should happen first and then routing.
But when we compare it to the OSI Model, Routing should happen first and then switching because the Network Layer (Layer 3) comes before the Data Link Layer (Layer 2, where the switches work).
I know there are 2 ways to look at the OSI Model. When we send data and when we receive data. Here I'm discussing the part of the OSI Model where the Data is sent.
Please clear my doubts, I will be glad if you did.
Thanks.

Ronnie Wong

@Atharva-Bet said in Doubt in understanding the application of OSI Model:

I know there are 2 ways to look at the OSI Model. When we send data and when we receive data. Here I'm discussing the part of the OSI Model where the Data is sent.
Please clear my doubts, I will be glad if you did.
Thanks.

Using the OSI model, when data is sent, the process of encapsulation occurs. That is, it adds headers to the data. So yes, the Layer 3 header is added before the Layer 2 header. But here's the issue. Encapsulation does not "move" the data along the data delivery path. The source encapsulation is key here because it adds the source IP, destination IP, source MAC, and destination MAC (if known).

When data is sent, you're not relying on encapsulation any more, you're now forwarding the data based on decapsulation.. This decapsulation occurs when the data is RECEIVED by the next forwarding device. So, when the source device doesn't know the destination MAC, it has to issue ARP--"Who is 192.168.10.112?" It looks for the hardware address of the device to send it to. But if that device is not on the network, who's MAC address will get applied? The router (default gateway) will reply with a proxy ARP-"I'm not 192.168.10.112, but use my MAC address, I can get it there." There are more details behind this but that should get you started.

So, yes, you're right in encapsulation the Layer 3 header (L3 PDU) is added before Layer 2 header (L2 PDU) but to send data (forward) data. It must send it a to device's a mac address - layer 2 first. Then that device can process the L3 PDU and verify that it as reach its destination or not. If it's a router, it probably has not reached its destination. When that happens, it means the router has to encapsulate a new L2 PDU towards the next device.