The answer is Layer 2, with the PDU being frames. A switch primarily operates at the Data Link layer of the OSI model because it makes forwarding decisions based on MAC addresses, not IP addresses, and the protocol data unit at this layer is the frame, which encapsulates the MAC header, payload, and trailer. For the CCNA 200-301 v2 exam, this concept tests your understanding of where switching occurs in the OSI stack and how to isolate issues like MAC table errors, VLAN mismatches, or duplex mismatches when frames are dropped. A common trap is confusing a switch’s Layer 2 operation with a router’s Layer 3 function, so remember that switches forward frames, not packets. Memory tip: “Layer 2 switches are frame fans—they love MACs, not IPs.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network technician is troubleshooting a connectivity issue where a host cannot communicate with a remote server. The technician notices that frames are being dropped at an intermediate switch. At which OSI model layer does the switch primarily operate, and what is the Protocol Data Unit (PDU) used at that layer?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Layer 2; frames
Switches primarily operate at Layer 2 (Data Link layer) of the OSI model, where they make forwarding decisions based on MAC addresses. The Protocol Data Unit (PDU) at this layer is the frame, which includes the MAC header, payload, and trailer. When frames are dropped at an intermediate switch, it indicates a Layer 2 issue such as a MAC address table problem, VLAN mismatch, or duplex mismatch.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Layer 1; bits
Why it's wrong here
Layer 1 (Physical) is concerned with raw bit transmission, not switching decisions based on MAC addresses.
✓
Layer 2; frames
Why this is correct
Switches operate at Layer 2 (Data Link) and use frames, which contain source and destination MAC addresses, to make forwarding decisions.
Related concept
Read the scenario before looking for a memorised answer.
✗
Layer 3; packets
Why it's wrong here
Layer 3 (Network) uses packets (or datagrams) for routing based on IP addresses, but switches typically do not perform routing.
✗
Layer 4; segments
Why it's wrong here
Layer 4 (Transport) uses segments (TCP) or datagrams (UDP) for end-to-end communication and is not involved in switch forwarding.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Layer 2; framesCorrect answer▾
Why this is correct
Switches operate at Layer 2 (Data Link) and use frames, which contain source and destination MAC addresses, to make forwarding decisions.
✗Layer 1; bitsWrong answer — click to see why▾
Why this is wrong here
Switches do not operate at Layer 1; hubs and repeaters do.
✗Layer 3; packetsWrong answer — click to see why▾
Why this is wrong here
While some multilayer switches can route, the basic switch in this scenario operates at Layer 2.
✗Layer 4; segmentsWrong answer — click to see why▾
Why this is wrong here
Segments are used by transport layer protocols, not by switches.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction that a standard switch operates at Layer 2, but candidates may incorrectly choose Layer 3 because they associate switches with VLANs or IP routing, forgetting that basic switching is a Layer 2 function.
Detailed technical explanation
How to think about this question
Switches maintain a MAC address table (CAM table) that maps MAC addresses to specific ports, learned by examining the source MAC address of incoming frames. When a frame arrives, the switch looks up the destination MAC address; if not found, it floods the frame out all ports except the ingress port. Frame drops can occur due to a full MAC address table, STP blocking a port, or a CRC error in the frame trailer. In a real-world scenario, a misconfigured trunk port with a native VLAN mismatch can cause frames to be dropped silently.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Layer 2; frames — Switches primarily operate at Layer 2 (Data Link layer) of the OSI model, where they make forwarding decisions based on MAC addresses. The Protocol Data Unit (PDU) at this layer is the frame, which includes the MAC header, payload, and trailer. When frames are dropped at an intermediate switch, it indicates a Layer 2 issue such as a MAC address table problem, VLAN mismatch, or duplex mismatch.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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