Q: Encapsulation

certskills
By certskills September 12, 2018 09:05

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).

#CLUS 2018 Note-taking Plan: Everlast Rocketbook and Livescribe 3
A: Encapsulation
certskills
By certskills September 12, 2018 09:05
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5 Comments

  1. Mit September 13, 07:42

    It should be R3 I guess.

    Reply to this comment
  2. Charles September 13, 09:18

    R3

    Reply to this comment
  3. nicholas September 13, 11:22

    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.

    Reply to this comment
    • CCENTSkills September 13, 11:36

      Nicholas,
      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.
      Wendell

      Reply to this comment
      • Nicholas September 13, 12:33

        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!
        thanks
        Nick

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