Layer 2 Frame/Packet

[bunjiega]

I have a basic funamental question that I can just not get my head around?!!

-On a layer 2 switch, frames are forwarded based on MAC addresses. However if you do a SPAN/Packet trace, you can see IP addresses going to and from the ports. I know switches do not route the packets, but I don't get why we see IP addresses going through a layer 2 switch. Looking at a frame:

There is no IP field. I'm not seeing where the layer 3 fits on layer 2 frame. What am I overanalyzing or overlooking?

[cadet alain · Accepted solution]

Hi,

the IP addresses are in the DATA portion of the ethernet frame structure you posted. But the L2 switch doesn't care about these infos because its only interested by L2 infos: source MAC and destination MAC

Regards.

Alain

[bunjiega]

Ok, that seems to make sense!

-From the PC perspective... if it is communicating with a remote device, it will put the source and destination IP addreses in the DATA field and when the router recieves that frame it will convert that frame into a packet, then continue on with L3.

-From the packet tracer perspective, it reads all the DATA fields of the frames that pass by and transform them to a "packet" and thows them onto the GUI so you can follow the layer 3 paths.

I don't recall ever reading that in any study materials that I have ever read. That concept has been a big hinderence to my furthering studies. Thanks a lot for the clarification!!