- A
Mismatched encryption algorithms
Why wrong: If encryption algorithms mismatched, the VPN tunnel would not establish successfully; the tunnel is up, so this is not likely.
- B
Asymmetric routing causing the stateful firewall to drop return traffic
The stateful firewall expects to see both directions of a connection. If traffic from Branch A enters one firewall interface but the return traffic leaves via a different path, the firewall sees the return packet as unsolicited and drops it.
- C
Incorrect DNS configuration
Why wrong: While DNS might cause name resolution issues, the traffic is being dropped by the firewall; ping by IP works to the gateway, so DNS is not the problem.
- D
MTU mismatch causing fragmentation issues
Why wrong: Fragmentation issues typically cause packet loss or timeouts, but the firewall would log drops due to stateful inspection, not usually due to MTU; MTU issues would not necessarily cause the firewall to drop traffic.
Quick Answer
The answer is asymmetric routing causing the stateful firewall to drop return traffic. This occurs because a stateful firewall maintains a session table that expects all packets in a given flow to traverse the same interfaces in both directions; when the VPN tunnel is up and the gateway responds to pings, Layer 3 connectivity is confirmed, but if the return path from Branch B’s servers takes a different route back to Branch A—perhaps through a separate ISP link or a different firewall interface—the firewall sees those return packets as unsolicited and drops them, even though the tunnel itself remains active. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how stateful inspection depends on symmetric traffic flows, and it’s a common trap where candidates focus on tunnel status or ACLs instead of the routing asymmetry. A quick memory tip: think “same path in, same path out” for stateful firewalls—if the return traffic takes a detour, the session table says “no way.”
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. 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 connectivity between two branch offices connected by a site-to-site VPN. The VPN tunnel shows as active and up. Users at Branch A can ping the VPN gateway IP at Branch B successfully, but they cannot access any servers behind the firewall at Branch B. The firewall at Branch B is stateful and its logs show that traffic from Branch A is being dropped. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
Asymmetric routing causing the stateful firewall to drop return traffic
The VPN tunnel is active and Branch A can ping the VPN gateway IP at Branch B, confirming that the tunnel itself and Layer 3 connectivity are functional. However, a stateful firewall tracks connection states based on source/destination IP and port; if traffic from Branch A enters the firewall on one interface but return traffic exits via a different path (asymmetric routing), the firewall sees the return packets as not belonging to any established session and drops them. This matches the log showing traffic being dropped despite the tunnel being up.
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.
- ✗
Mismatched encryption algorithms
Why it's wrong here
If encryption algorithms mismatched, the VPN tunnel would not establish successfully; the tunnel is up, so this is not likely.
- ✓
Asymmetric routing causing the stateful firewall to drop return traffic
Why this is correct
The stateful firewall expects to see both directions of a connection. If traffic from Branch A enters one firewall interface but the return traffic leaves via a different path, the firewall sees the return packet as unsolicited and drops it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Incorrect DNS configuration
- ✗
MTU mismatch causing fragmentation issues
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a 'green' tunnel status guarantees end-to-end application connectivity, but the trap here is that stateful firewalls require symmetric traffic flows, and candidates may incorrectly blame encryption mismatches or DNS when the tunnel itself is operational.
Detailed technical explanation
How to think about this question
Stateful firewalls maintain a session table (e.g., using 5-tuple: source IP, destination IP, source port, destination port, protocol). In asymmetric routing, return traffic may take a different physical path, causing the firewall to see SYN-ACK or data packets without a corresponding SYN entry, leading to a 'state violation' drop. This is common in multi-homed VPN designs or when route-based VPNs redistribute routes without proper policy-based routing (PBR) to enforce symmetry.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Asymmetric routing causing the stateful firewall to drop return traffic — The VPN tunnel is active and Branch A can ping the VPN gateway IP at Branch B, confirming that the tunnel itself and Layer 3 connectivity are functional. However, a stateful firewall tracks connection states based on source/destination IP and port; if traffic from Branch A enters the firewall on one interface but return traffic exits via a different path (asymmetric routing), the firewall sees the return packets as not belonging to any established session and drops them. This matches the log showing traffic being dropped despite the tunnel being up.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.
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