Question-CCNA level

Unanswered Question
Feb 14th, 2008

when a packet travels from one host to another crossing two routers and a switch, how many frames does it get encapsulated in? I think the answer should be 1 because the framing will only get done by the switch. But I'm not sure.. Can someone help plz...

I have this problem too.
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Istvan_Rabai Thu, 02/14/2008 - 13:31

Hi Uzma,

1. It is encapsulated in a frame from the host to the first router.

2. It is encapsulated in another frame when it leaves the first router to the second router.

It is not encapsulated by the layer2 switch. The switch switches the frame as it is.

3. It is encapsulated in a 3rd frame, when it leaves the second router.

No more encapsulations.



hang on

Switches encapsulate frames and routers encapsulate packets hence layer 2 and layer 3.

The PC will send the frame out as a layer 2 from its NIC. Switch passes to FA on the router which reads the destination network and encapsulates it in a packet and forwards to the next router.

This router looks for the destination and passes it out its fa as a l2 frame.

frame, frame, packet, frame.


uzmausmani Sat, 02/16/2008 - 14:32

Now I'm utterly confused or maybe not.. This is how I understood...

The PC encapsulates the data (1st frame encapsulation along the path) and forwards it onto the switch. The switch forwards the data to the default gateway (Router's FA interface). The Router encapsulates it into a packet (not a frame, as the question specifies) and forwards it onto the destination router which deencapsulates it and forwards the ethernet frame onto the host directly connected to the router. So my original answer of 1 frame encapsulation. obviously if ur answer of 3 frames is correct I'm going very wrong in my understanding of it all. Could u plz explain further. I would really appreciate it. Thanks..

uzmausmani Sat, 02/16/2008 - 15:17

Correction to above.. The answer should be 2 because first the data gets encapsulated into a frame at the sender and then it gets encapsulated again into a frame by the destination router which needs to add the layer 2 header before forwarding it to the receiver. why will the first router encapsulate it into a frame since its only forwarding it to another router through its serial port?

uzmausmani Sat, 02/16/2008 - 15:24

I'm sorry.. I got it now.. U're right.. Both routers will encapsulate it into a frame before sending it out!!! My bad! Thanks for your help..:)


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