A: Encapsulation

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

Can you predict the data link addresses in each frame that encapsulates a packet as it flows from source to destination in an IPv4 network? This latest QA requires you to think hard about those details. Check out the question before you click to see the answers below!

The Answers:



The Big Idea: Layer 3 Forwarding and De-encapsulation

Routers forward IP packets. As a part of that process, the router de-encapsulates the packet from the data link frame (that is, removes the packet), and discards the data link header and trailer. Then the router re-encapsulates the packet into a new data link frame, based on the data link protocol of the outgoing interface.

The question stem uses the alternate term “protocol data unit,” or PDU. Routers de-encapsulate the layer 3 PDU (L3PDU) from the layer 2 PDU (L2PDU) and then re-encapsulates the L3PDU into a new L2PDU.

The addresses in the data link header then have a specific job: cause the frame to be delivered to the next router, or to the destination host at the last step. You can see the data link addresses described here in figure 2, shown as they go left-to-right towards R1, R2, R3, and PC2. In each case, the data link headers’ destination addresses list the next device’s data link address.

Figure 2: Data Link Addresses PC1 Towards PC2


In the reverse direction, the same effect occurs. Figure 3 shows the details.


Figure 3: Data Link Addresses PC2-to-PC1


Why the Right Answer is Right

The question stem asks about a device that:

  • Discards a header (that is, performs de-encapsulation)
  • A data link header which lists PC2’s MAC as the Source MAC

As you can see from figures 2 and 3, the various frames (L2PDUs) have different combinations of source and destination MAC. However, only the frame sent by PC2, before R3 routes the packet that resides in the frame, uses PC2’s MAC as the source.

As for which router de-encapsulates the packet from that data link frame, R3 performs the de-encapsulation, before forwarding the frame listed as frame number 2 in figure 3.


Why the Answers about the Switches are Wrong

The figure shows two layer 2 switches, that is, two switches that do not perform routing, so they would never perform de-encapsulation and re-encapsulation of the IP packet. The reasons to know that the switches are layer 2 switches are:

  • The icons show layer 2 switch icons, rather than layer 3 switch icons.
  • The figure in the question post shows each PC, switch, and local router in the same subnet. If the switch were acting as a layer 3 switch, the PC would be in a different subnet than the router.

Figure 4 shows the layer 2 and layer 3 switch icons for comparison.

Figure 4: Layer 2 and Layer 3 Switch Icons

Q: Encapsulation
Q: LAN Broadcasts
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