OSI model & intermediate network Nodes

[ashish7aditya]

Dear Experts,

I have one query and need your comments..

As we know that based on OSI or TCP/IP models request goes from Top layer to bottom layer(where from it leaves the machine) and bottom to top layer at receiver end device.

But If I have in between nodes between two end devices, example

PC-----Switch ------Router-----Server,

PC want to ping Server, then in this process, will same concept of OSI 7 layers or TCP/IP layers will be followed on switch and router also ?

Meaning once the switch receive signal from PC, after that it will follow all the layers(Bottom to Top) and then will send signal to router by following ToP to bottom OSI model flow. And same concept to be followed on Router and Server also?

OR

it should be like this, Surely layer model will be followed by all the elements/nodes but based on the capabilities of intermediate device(Ex- S/R) it will read headers up to particular layer only. With this I mean, when 0101010... bits(signal) will reach Switch, it will first be mapped to L2 frame based on L2 protocol in use. Switch will read & modify L2 overheads but packet data unit will not be read or modified. and then new frame will leave the switch egress i/f and then again 010101100111101....... bits/ signal will reach router.

Router being intelligent to read & understand packet, it will read packet header and will act accordingly but will not read or modify anything on data segment unit. and then accordingly it will define new frame @L2 and then finally bits at Phy layer will leave router egress i/f.

Finally once signal 01010010..... will reach server, it will go through all the layers(bottom to top).

Summary: layer model will be followed by all the networking devices. Only thing is that based on the device capability it will read / modify data units up to particular layer. Like pure L2 switch will not read any thing in packet and router will not read transport data segments and so on...

Is this understanding correct ?

[M. G. · Accepted solution]

Hi, 

You are right, layered model will be followed all the way but in this case up to second layer on the switch and up tothird layer on the router.. The destination IP address will stay the same all the time, MAC address will be changed over the route.

In your case,  if you ping Server from PC it will be something like this:

From PC it will start with ICMP packet + Eth Frame (dst IP =Server IP, dst MAC= Router MAC)--- switch will forward the packet to the router  based on L2 information ( it will not look upper layers as it has no capability nor need to do so). Once the packet reaches router, router will forward the packet on the network where the destintaion is and then the MAC address will be used to reach the server on that network. 

Routers and swithces never look upper layers as they don't really care what is inside. IP address enables the packet to be forwarded toward its destination, MAC address enables the packet to be carried by the local media across each segment.

Cheers,