CCNA Network Infrastructure and Connectivity Practice Question
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?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
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
The Physical layer (Layer 1) handles raw bit transmission—encoding, signaling, and media characteristics—but it has no mechanism to interpret MAC addresses. A switch receives bits at the physical layer, yet it must reassemble them into a frame and examine the destination MAC address before deciding where to forward. Thus 'Layer 1; bits' fails because the forwarding decision is made after L2 header parsing, not on individual bit values.
- ✓
Layer 2; frames
Why this is correct
Switches are Layer 2 devices that forward frames. When a frame arrives, the switch parses its Data Link header, reads the destination MAC address, and looks up that address in its MAC address table to determine the egress port. It then forwards the entire frame out the appropriate interface (or floods it if the address is unknown), making 'Layer 2; frames' the correct answer.
- ✗
Layer 3; packets
Why it's wrong here
Layer 3 (Network) involves packets and IP addressing, which is the domain of routers, not typical switches. Standard switching does not inspect the IP header or use the destination IP address to make forwarding decisions; it relies exclusively on MAC addresses carried in the frame. Even though multilayer switches can perform routing, the fundamental forwarding operation of a switch is still frame-based at Layer 2, not packet-based.
- ✗
Layer 4; segments
Why it's wrong here
The Transport layer (Layer 4) manages end-to-end communication using segments, sequence numbers, ports, and acknowledgments—functions handled by hosts, not switches. A switch does not read TCP or UDP headers, track session state, or process port numbers when forwarding traffic. Transport-layer segments remain encapsulated within packets and frames, so the switch forwards the frame without ever examining Layer 4 information.
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 are Layer 2 devices that forward frames. When a frame arrives, the switch parses its Data Link header, reads the destination MAC address, and looks up that address in its MAC address table to determine the egress port. It then forwards the entire frame out the appropriate interface (or floods it if the address is unknown), making 'Layer 2; frames' the correct answer.
✗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?”
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
Go deeper
Related to this question
Learn chapter
The OSI Model
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
PDU
A PDU (Protocol Data Unit) is the unit of data at a specific layer of the OSI or TCP/IP model, containing both header and payload information as it travels across a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.