Q: Encapsulation

 In 200-301 V1 Ch04: Command Line, 200-301 V1 Part 2: Ethernet, CCENT-OLD, Q&A

Time for some new practice questions! I’ve not added any new QA in a while, but I plan to get on a roll and add new QA to both the CCENT blog and CCNA blog for a good run of months. The plan: work through each of the books (the ICND1 and ICND2 Cert Guides) from front-to-back, adding questions as I go.

First up: A question about layers and encapsulation.

The Question

PC1 issues a ping command for PC2, and it works. Which devices de-encapsulates a layer 3 PDU, discarding a header that shows a source MAC address of PC2’s MAC?

A) SW1

B) R1

C) R2

D) R3

E) SW2

Figure 1: Figure Reference

Answer Post in a Few Days!

I’ll post an answer post in a few days. Watch for it! It should be the next post in chronological order as well, so watch for the link at the bottom of this page (just above the comments section).

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A: Encapsulation
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It should be R3 I guess.




I would guess R3 or D also. the only thing that concerns me is the wording. I thought that routers DE-encapsulate layer TWO PDU’s, throwing away the dest and source macs, check the routing table then RE-encapsulating a layer 3 pdu in a layer 2 for the routers outgoing interface.


Good point about the terms and wording – terms and meaning can often be the issue more than your understanding of of the bits/bytes.
Routers de-encapsulate the layer 3 PDU from the layer 2 PDU. Focusing on the words, “… routers de-encapsulate…” means “… routers remove these bytes from some other larger set of bytes…”. So the bytes removed are the L3PDU. The de-encapsulation process discards the now unneeded L2 header/trailer.
Likely that we’re describing the same ideas with slightly different English wording.


Thanks Wendell,
Always enjoy your books and blogs! They are all I used for 100-105 and I passed easily the first time. I am curently studying for 200-105 and am up to ch7 OSPF. if you have time, I understand the concept of the DR and the BDR but I’m having trouble understanding a case scenario where you would have 5 routers on the same subnet. Any explanition or direction to an alternate source is appreciated!

Ramanathan Prabakaran

I agree that unlike other Faculties such as Arts, or, commerce, the beauty, or quality of science, and technology is the details should be very precise, and accurate. Sometimes virtually it is not possible to represent or explain exactly the point or subject in language. Briefly saying it is tricky.

Punya Atma

The answer is, choice (D), R3.

Source mac address of PC2, is used in the Ethernet header of the Echo reply
Layer 3 PDU message from PC2, for the Echo request message of PC1. So in the return path, this Echo reply message first switches out of SW2 port to R3.
So here it is the Router R3, de-encapsulates the Ethernet header of the L3 PDU message from PC2, and replaces the source mac with its own source mac and destination mac with R2’s mac address.




D) R3

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