- A
The problem is at the Transport layer (Layer 4); successful operations are at the Application layer (Layer 7) only.
Why wrong: This is incorrect because the TCP connection was successfully established, indicating no Transport layer problem. The error is an application-layer response.
- B
The problem is at the Application layer (Layer 7); successful operations are at the Application layer (Layer 7) and Transport layer (Layer 4).
DNS resolution (Layer 7) and TCP connection (Layer 4) are successful. The sending error is an Application layer issue, as the mail server returns an application-level error.
- C
The problem is at the Network layer (Layer 3); successful operations are at the Data Link layer (Layer 2) and Physical layer (Layer 1).
Why wrong: This is incorrect because the technician observed TCP connection establishment, which requires Network layer IP routing to be functional. The problem is not at Layer 3.
- D
The problem is at the Presentation layer (Layer 6); successful operations are at the Session layer (Layer 5) and Transport layer (Layer 4).
Why wrong: This is incorrect because the error is an application-layer response, not a data format or encryption issue (Presentation) or session management issue (Session).
OSI Application Layer Troubleshooting: Email Client Issues
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 user's email client cannot send messages, but the client can receive emails. The technician uses a protocol analyzer and sees that the client is successfully resolving the mail server's domain name to an IP address and establishing a TCP connection, but the server responds with an application-layer error. At which layers of the OSI model are the problem and the successful operations occurring, respectively?
Quick Answer
The answer is the Application layer (Layer 7) for the problem, with successful operations at both the Application layer (Layer 7) and the Transport layer (Layer 4). This is correct because the email client successfully resolves the mail server’s domain name via DNS—a Layer 7 protocol—and completes a TCP three-way handshake at Layer 4, yet the server returns an application-layer error such as an SMTP 550 or 554 code. Since the underlying network and transport functions are intact, the failure must stem from the email protocol itself, like authentication rejection or a full mailbox, which resides solely at Layer 7. On the CCNA 200-301 v2 exam, this scenario tests your ability to isolate OSI application layer email troubleshooting by recognizing that DNS resolution and TCP connectivity do not guarantee successful email delivery; a common trap is assuming a TCP connection implies the application is working. Remember the mnemonic “DNS and TCP are fine, but SMTP says decline” to keep Layer 7 as your focus when the server sends a protocol-level error.
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
The problem is at the Application layer (Layer 7); successful operations are at the Application layer (Layer 7) and Transport layer (Layer 4).
The problem is at the Application layer (Layer 7) because the email client can resolve the domain name (DNS, Layer 7), establish a TCP connection (Transport layer, Layer 4), but the mail server returns an application-layer error (e.g., SMTP 550 or 554), indicating the issue lies in the email protocol itself (e.g., authentication failure, mailbox full, or rejected sender). Successful operations include DNS resolution (Application layer) and TCP three-way handshake (Transport layer), confirming layers 7 and 4 are functioning correctly.
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.
- ✗
The problem is at the Transport layer (Layer 4); successful operations are at the Application layer (Layer 7) only.
Why it's wrong here
This is incorrect because the TCP connection was successfully established, indicating no Transport layer problem. The error is an application-layer response.
- ✓
The problem is at the Application layer (Layer 7); successful operations are at the Application layer (Layer 7) and Transport layer (Layer 4).
- ✗
The problem is at the Network layer (Layer 3); successful operations are at the Data Link layer (Layer 2) and Physical layer (Layer 1).
Why it's wrong here
This is incorrect because the technician observed TCP connection establishment, which requires Network layer IP routing to be functional. The problem is not at Layer 3.
- ✗
The problem is at the Presentation layer (Layer 6); successful operations are at the Session layer (Layer 5) and Transport layer (Layer 4).
Why it's wrong here
This is incorrect because the error is an application-layer response, not a data format or encryption issue (Presentation) or session management issue (Session).
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.
✓The problem is at the Application layer (Layer 7); successful operations are at the Application layer (Layer 7) and Transport layer (Layer 4).Correct answer▾
Why this is correct
DNS resolution (Layer 7) and TCP connection (Layer 4) are successful. The sending error is an Application layer issue, as the mail server returns an application-level error.
✗The problem is at the Transport layer (Layer 4); successful operations are at the Application layer (Layer 7) only.Wrong answer — click to see why▾
Why this is wrong here
The TCP handshake succeeded, so the Transport layer is working. The error message is generated by the application, not the transport protocol.
✗The problem is at the Network layer (Layer 3); successful operations are at the Data Link layer (Layer 2) and Physical layer (Layer 1).Wrong answer — click to see why▾
Why this is wrong here
Successful TCP connection implies IP routing (Layer 3) is working. The error is not related to addressing or routing.
✗The problem is at the Presentation layer (Layer 6); successful operations are at the Session layer (Layer 5) and Transport layer (Layer 4).Wrong answer — click to see why▾
Why this is wrong here
Email sending failures are typically application logic errors, not encryption/formatting or session problems. Also, DNS is an Application layer protocol, not Session or Presentation.
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 between successful lower-layer operations (DNS, TCP) and an application-layer failure, trapping candidates who assume any email problem must be at the Transport or Network layer because they confuse 'connection established' with 'protocol function working'.
Detailed technical explanation
How to think about this question
In SMTP email delivery, the client sends a MAIL FROM command followed by RCPT TO and DATA; an application-layer error like '550 5.1.1 User unknown' or '554 5.7.1 Relay access denied' is generated by the mail server's MTA (Message Transfer Agent) after the TCP connection and SMTP greeting (220) succeed. This highlights that the OSI model's Application layer encompasses protocols like SMTP, POP3, IMAP, and DNS, and errors at this layer are distinct from lower-layer connectivity issues.
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 practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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 |
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
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: The problem is at the Application layer (Layer 7); successful operations are at the Application layer (Layer 7) and Transport layer (Layer 4). — The problem is at the Application layer (Layer 7) because the email client can resolve the domain name (DNS, Layer 7), establish a TCP connection (Transport layer, Layer 4), but the mail server returns an application-layer error (e.g., SMTP 550 or 554), indicating the issue lies in the email protocol itself (e.g., authentication failure, mailbox full, or rejected sender). Successful operations include DNS resolution (Application layer) and TCP three-way handshake (Transport layer), confirming layers 7 and 4 are functioning correctly.
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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Last reviewed: Jun 11, 2026
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