Switch Learning Vs. Switch Forwarding Question

 In 200-301 V1 Ch05: Ethernet Switching, 200-301 V1 Part 2: Ethernet, CCENT-OLD, Q&A

For #CCENT and #CCNA, you need to know how switches learn, plus you need to know how switches forward frames. Sometimes, combining the two concepts (learning and forwarding) for the first time causes some mental hurdles.

In today’s post, we’ll use one of the questions from my ICND1 Cert Guide as a backdrop to work through these same concepts. The question weaves together learning and forwarding. Today’s post poses the question, while the next post works through the logic.

The Diagram

Figure 1: Diagram Used with this Question

The Question

The switch begins with no dynamically learned MAC addresses. The PC1 creates and sends a frame with a destination address of 2222.2222.2222. Then PC3 creates and sends a frame destined for PC2’s MAC address of 2222.2222.2222. Which of the following are true about this second frame? (Select all answers that are correct.)

The Possible Answers

A.      The switch discards (filters) the frame.

B.      The switch forwards the frame out of interface Fa0/1

C.      The switch forwards the frame out of interface Fa0/2

D.      The switch forwards the frame out of interface Fa0/3

Answer: Overlapping Connected and Routing Protocol Routes
Switch Learning Vs. Switch Forwarding - Analysis
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Toigonbai

C. The switch forwards the frame out of interface Fa0/2

bars_dnb

After PC1’s frame switch only learned PC1’s MAC (if there was no reply from PC2, which is not implied i guess). So when PC3 sends a frame destined for PC2’s MAC – switch doesn’t know where to send a frame, so it sends it out of all up interfaces except incoming. Answers: B and C.

artem718

the answer is C because the first frame would be flooded out all the ports except the f0/1, 2nd frame will be forwarded out of f0/2 because the CAM table has learned the mac addresses of
PC1 and PC2

lyjo

Thanks!
Check out the answers post if interested… just click the link above the comments but below the post.
Wendell

[…] The original question […]

[…] The original question […]

[…] A related switch forwarding question […]

Brenden

The key to answer this questions is to understand that hundreds or thousands of frames can arrive at the switch at different time. So this question specifically asking about the 2nd frame which the assumption that frame will be delivered first. Since the switch has knowledge of the MAC table, it will flood to all ports except the incoming port.

Raam Prabha

answers B and C

Punya Athma

At the start the frame from PC1 enters Switch’s Fa0/1 port. SW1 learns PC1’s MAC address and put this on its already empty MAC address table. As this frame is an unknown uni cast frame SW1 floods out this frame via the switch’s Fa0/2 and Fa0/3 ports. Then the frame from PC3 destined to PC2, but it is also an unknown uni cast frame for the Switch as PC2’s MAC address is not in the Switch MAC table. So the Switch floods this frame via Switch ports Fa0/1, and Fa0/2.

Valentí

Answer: B, C

The second frame will be forwarded to all ports (Fa0/1, Fa0/2) of the switch except the source port (Fa0/3), because SW1 does not know who PC2 is (it does not have the MAC 2222.2222.2222 in its table)

Wendell Odom

Hi Valenti,
I agree!
Thanks for all the comments on the answers in the last week!
Wendell

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