Answer: Forwarding and Encapsulation

CCENTSkills
By CCENTSkills January 22, 2018 14:05

The #CCNA exam can pack a punch with what looks like a simple question, and this latest practice question is just such an example. The question asks about switch forwarding, but it combines packet encapsulation concepts. Get either wrong, and you may get the question incorrect. I kept this one to a relatively straightforward case, with a plan to add another question or two about this topic over time. I’ll also explore it in my upcoming live course at SafariBooksOnline. In this meantime, here’s the answer and explanation!

Answer

D: One of R2’s interface MAC addresses

 

Short Explanation

The question asks about switch SW2’s forwarding logic, but the question does not describe the destination MAC address or the VLAN information, which are the two facts used by a layer 2 switch when forwarding a frame. The answers all refer to the address used, and ignores the VLANs, so to answer the question, you first need to determine the destination MAC address in the frame at the point the frame arrives at switch SW2.

In this case, the question describes a message sent by server B to host A, so the message has traveled only from server B over the link to switch SW2. The sending host, server B, sends the message with these addresses:

  • Host B’s IP address as the source IP address
  • Host A’s IP address as the destination IP address
  • Host B’s MAC address as the source MAC address
  • Router R2’s G0/2 interface’s MAC address as the destination MAC address

Server B uses router R2’s G0/2 interface MAC address as the destination MAC address because R2’s G0/2 interface IP address will be server B’s default gateway. To send the IPv4 packet to a host in another subnet, server B forwards the packet to its default gateway. To do so, server B encapsulates the IP packet in an Ethernet frame, with the default gateway’s MAC address as the destination MAC address.

As for the specific answers:

Two answers refer to IP addresses; layer 2 switches do not forward messages based on an IP address at all, so those two answers can be ruled out. As for the four answers that mention a MAC address, only one specifies a MAC address on router R2. Most importantly, with server B sending an IP packet to host A, which resides in a different subnet, server B does not use host A’s MAC address as the destination MAC of the frame.

 

Common Mistakes

To close this post, let me give you a few more pointers that may help you avoid losing points on the exam:

The figure uses the icon for a layer 2 switch. The question does not state whether the switch is a layer 2 or layer 3 switch, but if it doesn’t tell you in some way, and you see the layer 2 switch icon as shown here, think layer 2 switch.

This question shows an example of how you can rule out some answers quickly based on general theory rather than the specific scenario. Layer 2 switches do not forward messages based on layer 3 addresses, so you could have quickly ruled out the two answers that claim the switches use IP addresses to make their forwarding decisions.

Question: Forwarding and Encapsulation
Predicting ARP Messages 1
CCENTSkills
By CCENTSkills January 22, 2018 14:05
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6 Comments

  1. theurge14 January 24, 23:15

    Thanks for this, since I’m in the middle of Chapter 7 of your CCENT book I’m so focused on the frames I kinda overlooked the routers in the diagram at first! This is a good reminder to slow down and break down the encapsulation and deencapsulation process step by step on the diagram using the forwarding/filtering switch logic.

    Reply to this comment
    • CCENTSkills Author January 25, 06:47

      You’re welcome! You’ll be heads-down on Switching for a few more chapters, so I agree, great timing to take a step back. Thanks for the note!

      Reply to this comment
  2. kcn38Si February 10, 03:45

    Just to let you know this answer page is tagged to “Chapter 17” menu incorrectly. The original question is in Chapter 7, and this is where this answer should belong. Thanks!

    Reply to this comment
    • CCENTSkills Author February 12, 10:26

      Thanks for the note. FYI, I marked it with Chapter 18 and Part V on purpose, as the answer makes some reference to ARP as used on the routers. (I had marked Chapter 17 incorrectly, but it’s now fixed to Chapter 18). Regardless, thanks or the note – it’s hard to get all these details straight!

      Reply to this comment
  3. CDcroc May 17, 20:32

    My mind went multiple choice and I chose B and D. So I understand why D is correct but why is B wrong? Wouldn’t Host B’s MAC address be in SW2’s CAM?

    Reply to this comment
    • CCENTSkills Author May 24, 07:51

      Hi CDcroc,
      To your specific question, the answer is no. Look at the figure, where it shows the addresses (well, the descriptions of the addresses). The question asks which address(es) SW2 uses to forward the frame. IT will use the destination MAC, which is an address on R2. Answer B, which is “Host B’s MAC”, is the source MAC. That address is not used by SW2 to make the forwarding decision.
      Hope this helps…
      Wendell

      Reply to this comment
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