The answer is a WAN interface ACL blocking ESP traffic. When an IPsec VPN tunnel is up but traffic is not passing, the most likely cause is that the access control list on the WAN interface is dropping encrypted packets, specifically ESP (protocol 50) or UDP/4500 for NAT-T. Even though IKE and IPsec security associations are established—making the tunnel appear operational—the ACL silently discards the encrypted data, preventing any actual data flow. On the Cisco SCOR 350-701 exam, this scenario tests your ability to distinguish between tunnel establishment issues and traffic-forwarding issues; a common trap is assuming a transform set or IKE mismatch, but those would prevent the tunnel from coming up at all. The key memory tip is “Tunnel up, no traffic? Check the ACL—ESP must be permitted.”
350-701 Security Concepts Practice Question
This 350-701 practice question tests your understanding of security concepts. 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.
An engineer is troubleshooting an IPsec VPN between two Cisco routers. The tunnel is up, but traffic is not passing. The encryption domain on both sides is correctly configured. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ACL on the WAN interface blocking ESP traffic
When the IPsec tunnel is up but no traffic passes, the most common cause is that the WAN interface ACL is blocking ESP (protocol 50) or UDP/4500 (NAT-T) traffic. Even though IKE and IPsec SAs are established, if the ACL drops the encrypted packets, the tunnel appears operational but cannot forward data. This is distinct from transform set or IKE mismatches, which would prevent the tunnel from coming up at all.
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 IPSec transform sets
Why it's wrong here
If transform sets mismatched, the IPsec SA would not be established.
✗
Routing loop
Why it's wrong here
A routing loop is unlikely if the tunnel is up; routing should work normally.
✓
ACL on the WAN interface blocking ESP traffic
Why this is correct
ESP traffic (IP protocol 50) may be dropped by an inbound or outbound ACL.
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.
✗
Mismatched IKE phase 1 parameters
Why it's wrong here
If IKE parameters mismatched, the tunnel would not come up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between tunnel establishment (IKE phase 1 and 2) and data-plane forwarding, tricking candidates into thinking a tunnel being 'up' guarantees traffic flow, when in fact ACLs or firewall rules can block the encrypted payload.
Detailed technical explanation
How to think about this question
IPsec VPNs use ESP (protocol 50) or AH (protocol 51) for encrypted data, and these are not TCP/UDP-based, so standard ACLs must explicitly permit them. Additionally, if NAT traversal is in use, UDP port 4500 must be allowed. A common real-world scenario is when an ACL is applied inbound on the WAN interface that implicitly denies ESP, causing the tunnel to establish (since IKE uses UDP/500) but data packets to be silently dropped. The 'show crypto ipsec sa' command would show packets encaps/decaps counters incrementing only on one side, or not at all.
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.
Security Concepts — This question tests Security Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ACL on the WAN interface blocking ESP traffic — When the IPsec tunnel is up but no traffic passes, the most common cause is that the WAN interface ACL is blocking ESP (protocol 50) or UDP/4500 (NAT-T) traffic. Even though IKE and IPsec SAs are established, if the ACL drops the encrypted packets, the tunnel appears operational but cannot forward data. This is distinct from transform set or IKE mismatches, which would prevent the tunnel from coming up at all.
What should I do if I get this 350-701 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 350-701 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 350-701 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.