Which is the largest entity?

[swaminathan]

Hi

In OSI model, which is the largest entity related to the question below.Can anybody give clear explanation?

for the question I found in one CISCO LITREATURE as follows:

The question is:
Imagine that PC1 needs to send some data to PC2, and PC1 and PC2 are separated by
several routers. What are the largest entities that make it from PC1 to PC2?
a. Frame
b. Segment
c. Packet
d. L5 PDU
e. L3 PDU
f. L1 PDU

The answer for the question is c& e

Thanks and regards

S.Swaminathan

[Big Evil]

Funny enough i was reading on Cisco learning network the other month and saw a great answer by Keith Barker.

https://learningnetwork.cisco.com/thread/27943

[swaminathan]

Hi

What about this link in cisco https://learningnetwork.cisco.com/message/54237#54237

Where I found that as follows in that post.

"However, when it comes to the size of the data chunk, wouldn't 'A. Frame' be a better choice? I think the question is a little vague considering 'the largest entity in size' is indeed frame (as frame has an extra header and trailer than packet)."


I saw already the link which you have posted but I have the same opinion as quoted above in the red

Any explanation?

Thanks & Regards

S.Swaminathan

[omd]

A frame is indeed a larger entity than the packet it encapsulates in terms of its size in bytes. However, frames do not traverse routers. So, the frame that arrives at the final destination is not the same frame that was put onto the wire by the original source. The packet is the largest entity that moves from PC1 to PC2.

Assuming there is just one router between PC1 and PC2...

The frame that arrives at the router will have a header containing the destination mac address of the router interface on which the frame was received and the source mac address of PC1. The encapsulation will be removed, leaving just the Layer 3 packet, identical to what was formed at PC1. Upon examining the Layer 3 destination address, the router will give the packet new framing wherein the header will have the source mac address of the router's outgoing interface and the destination mac address of PC2. A new checksum will be computed and tacked on.

Strictly speaking, I would say that it is incorrect to say that the frame's header is modified as it goes through the router. It is more accurate to say that the framing/encapsulation from PC1's LAN is removed, and the packet is then encapsulated with framing appropriate for PC2's LAN. Don't forget that PC1's LAN and PC2's LAN could be completely different technologies; ethernet, firewire, ppp over RS-232 and so on. So the data structures in the encapsulation headers would be different for each of these.

Remember, Layer 2 is used to move things between two nodes (assuming a unicast) that are directly attached to the same LAN or point-to-point link; no further. Crossing over from one LAN or point-to-point link to a different LAN or point-to-point link is a Layer 3 activity.

[swaminathan]

Hi Omd

Clear and precise explanation given by you.Thankyou very much

Regards

S.Swaminathan