How a Switch Handles an ARP Request for an Unknown Destination

200-301 By islever Editorial Last updated

Describe switching concepts

Short answer

A An ARP request is a broadcast frame, so S1 floods it out every port except the receiving port G0/0 until PC3's MAC is learned.

Question

Refer to the exhibit. PC1 tries to ping PC3 for the first time and sends out an ARP to S1. Which action is taken by S1? - Exhibit: How a Switch Handles an ARP Request for an Unknown Destination
  1. It is flooded out every port except G0/0. Correct Answer
  2. It drops the frame.
  3. It forwards it out G0/3 only.
  4. It forwards it out interface G0/2 only.

Key Takeaways

ARP is a Layer 2 broadcast frame to the switch, not a Layer 3 routed packet — the switch floods it; it does not selectively forward it toward a gateway.

An ARP request uses destination MAC ffff.ffff.ffff, a Layer 2 broadcast. A switch cannot route it; it floods broadcasts within the VLAN out all ports except the ingress port. The "first time" detail means S1 has no MAC entry for PC3 yet, reinforcing the flood.

Common mistake: Assuming the switch forwards the ARP only to G0/2 or G0/3 (C/D) or drops it (B). A switch floods unknown/broadcast frames; it does not decide the path from the IP address.

Expert Analysis

Why the Answer Is Correct

A is correct. PC1's ARP request is a broadcast (destination MAC ffff.ffff.ffff). S1, having no entry for PC3 yet (this is the "first time"), floods the frame out every port in the VLAN except the receiving port G0/0. Flooding is the defined switch behavior for broadcast and unknown-unicast frames.

Why the Other Options Are Wrong

B is wrong: a switch does not drop a valid broadcast. C and D are wrong: the switch does not forward a broadcast only to a specific access port based on the IP — broadcasts go to all ports in the VLAN except ingress. Some comments mistakenly treat ARP as Layer 3 routed to the gateway, but to the switch it is a Layer 2 broadcast frame.

Community Comment Notes

The leading comment (75 votes for A) reasons that because PC1 sent an ARP, S1 must lack a PC3 entry, so it floods. A minority argues C, confusing ARP (Network layer) with switch forwarding, but the switch acts on the L2 broadcast.

Official Reference

https://learningnetwork.cisco.com/s/ccna-exam-topics

This link opens the official vendor documentation in a new tab.

Community Votes

A
75%
C
25%

75% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. Aleksandre 👍 3 Answered A
    1)PC1 tries to ping PC3 for the first time. 2)sends out an ARP to S1. If SW1 were to have an entry for PC3 in it's table, PC1 would not have had sent an arp request, that means that SW1 has no entry for PC3, so it will be flooded out of every port, answer is A.
  2. Rolfer 👍 2
    Very weird question, but it made me wonder some interesting questions. Please, correct me if im wrong. - Is S1's ARP table supposed to be empty? If so, answer A would be correct, but it does not indicate that the ARP table is empty. The statement says that PC1 tries to ping PC3 for the first time, but we do not know if S1 already knows PC1's MAC because it has previously sent traffic to other devices, nor do we know if S1 has information from PC3 for the same reason. - Are the PCs supposed to be on different VLANs? It is assumed that it is, but it is not specified. - It is assumed that if they are not on the same VLAN, it is a "Router on a Stick so: In theory, if the PCs are not in the same VLAN, PC1 would directly send the frame to the default gateway on Router, because PC1 knows that PC3 is not in the same VLAN by looking at its own network mask. In any case, the key would be to know if S1's ARP table is empty or not. Information is clearly missing from the question.
  3. resoluteq 👍 1 Answered C
    it says ARP request. ARP works at Network Layer and is not handled by switch, instead the switch forwards the frame containing ARP request to the default Gateway which is probably living in the router.
  4. tumajay 👍 1 Answered A
    The switch will first perform frame flooding on layer2 before it gets to layers arp
  5. 593554d 👍 1
    ARP src pc1 Destination MAC address: ffff:ffff:fff
Show 2 more comments
  1. hands 👍 1 Answered C
    Is asking about ARP, not about mac learning
  2. bymrdas 👍 1 Answered A
    A is correct

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