CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements are correct regarding Protocol Data Units (PDUs) and data encapsulation in the OSI model?
⚠ Common exam trap
Cisco often tests the direction of encapsulation (down the stack) versus de-encapsulation (up the stack), and the specific PDU names at each layer, to catch candidates who confuse these fundamental concepts.
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
✓
Data is encapsulated with a TCP header at the Transport layer to form a segment.
Option B is correct because at the Transport layer, application data is encapsulated with a TCP header (or UDP header) to form a segment, which is the Transport-layer PDU. Option C is correct because the Network layer PDU is the packet, formed by adding an IP header to the segment, providing logical addressing and routing information. Option A is incorrect because a frame is the Data Link layer PDU, not the Transport layer PDU. Option D is incorrect because the Data Link layer adds a header and trailer to create a frame, not a bit; bits are the Physical layer PDU. Option E is incorrect because encapsulation occurs as data moves down the OSI layers from Application to Physical, not upward.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The PDU at the Transport layer is called a frame.
Why it's wrong here
Calling the Transport layer PDU a frame is incorrect because the transport PDU is a segment (with TCP) or a datagram (with UDP). The frame is exclusively the Data Link layer PDU, created when the packet is wrapped with L2 header and trailer. Misnaming PDUs commonly leads to confusion between the functions of the transport and data link layers.
- ✓
Data is encapsulated with a TCP header at the Transport layer to form a segment.
Why this is correct
At the Transport layer, the protocol adds a TCP or UDP header to the application data, and when TCP is used, the resulting unit is called a segment. This process is part of encapsulation, where the upper-layer payload is wrapped with control information to enable reliable or best-effort delivery between ports. The segment is then handed down to the Network layer for IP encapsulation.
- ✓
The Network layer PDU is the packet, which includes the IP header.
Why this is correct
At the Network layer, the protocol data unit is the packet. This PDU is constructed by taking the upper-layer segment or datagram and prefixing an IP header, which contains source and destination IP addresses, TTL, protocol type, and other fields. Because the IP header is the defining component, a packet is specifically the L3 PDU in IPv4/IPv6 networks.
- ✗
The Data Link layer adds a header and trailer to create a bit.
Why it's wrong here
The Data Link layer adds a header and trailer to the Network layer packet, but the resulting PDU is a frame, not a bit. A bit is an individual electrical or optical pulse and is the Physical layer PDU. The trailer contains the frame check sequence for error detection, which is why the Data Link layer produces a complete frame.
- ✗
Encapsulation occurs as data moves up the OSI layers from Physical to Application.
Why it's wrong here
Encapsulation happens when data is transmitted from source to destination; it wraps data as it moves down the layers (Application -> Presentation -> Session -> Transport -> Network -> Data Link -> Physical). Decapsulation occurs moving up.
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.
✓Data is encapsulated with a TCP header at the Transport layer to form a segment.Correct answer▾
Why this is correct
At the Transport layer, the protocol adds a TCP or UDP header to the application data, and when TCP is used, the resulting unit is called a segment. This process is part of encapsulation, where the upper-layer payload is wrapped with control information to enable reliable or best-effort delivery between ports. The segment is then handed down to the Network layer for IP encapsulation.
✗The PDU at the Transport layer is called a frame.Wrong answer — click to see why▾
Why this is wrong here
This statement incorrectly identifies the Transport layer PDU; it is actually a segment or datagram, while a frame is at Layer 2.
✗The Data Link layer adds a header and trailer to create a bit.Wrong answer — click to see why▾
Why this is wrong here
Bits are created at the Physical layer, not the Data Link layer; the Data Link layer creates frames.
✗Encapsulation occurs as data moves up the OSI layers from Physical to Application.Wrong answer — click to see why▾
Why this is wrong here
This describes decapsulation, not encapsulation. Encapsulation occurs when moving down the OSI model stack.
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?”
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
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Key term
Media Access Control
Media Access Control (MAC) is a sublayer of the Data Link Layer in networking that controls how devices on the same network share access to the physical medium and uniquely identifies each device with a hardware address.
Key term
Encapsulation
Encapsulation is the process of wrapping data with protocol headers and trailers before sending it 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.