The answer is that the ACL for interesting traffic only permits ICMP, so only ICMP is encrypted. In Cisco IOS IPsec VPN configuration, the crypto map ACL defines which traffic is considered "interesting" and thus triggers encryption; if the ACL only contains a permit statement for ICMP (e.g., `permit icmp any any`), then only ICMP packets will be encrypted, while all other traffic like HTTP is forwarded in clear text because it does not match the ACL and is not processed by IPsec. This scenario is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding that the ACL does not filter traffic for security but rather selects which traffic to protect—a common misconfiguration where engineers assume a broad ACL is applied but inadvertently narrow it. Remember: the crypto map ACL is the gatekeeper for encryption; if it only says "ICMP," only pings get the tunnel. Memory tip: "Interesting traffic is selected traffic—if your ACL is too specific, your encryption is too limited."
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures an IPsec VPN between two routers using a transform-set with ESP encryption (AES 256) and ESP authentication (SHA-256). The engineer also applies an IPv4 ACL to define interesting traffic that matches all IP traffic. After configuration, the VPN tunnel comes up, but only ICMP traffic is encrypted; other traffic like HTTP is sent in clear text. What is the most likely explanation?
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
✓
The ACL for interesting traffic only permits ICMP, so only ICMP is encrypted.
The most likely explanation is that the ACL for interesting traffic only permits ICMP, so only ICMP is encrypted. In Cisco IOS, the crypto map uses the ACL to identify which traffic should be protected by IPsec. If the ACL only matches ICMP (e.g., 'permit icmp any any'), then only ICMP packets trigger IPsec encryption; all other traffic, such as HTTP, is forwarded in clear text because it does not match the ACL and is therefore not subject to IPsec processing.
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 ACL for interesting traffic only permits ICMP, so only ICMP is encrypted.
Why this is correct
IPsec uses the ACL to determine which traffic to protect; if only ICMP is permitted, other traffic is not encrypted.
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.
✗
The transform-set does not support HTTP traffic due to port filtering.
Why it's wrong here
Transform-set defines encryption and authentication algorithms, not traffic selection.
✗
The IPsec SA is only established for ICMP; other traffic requires a separate SA.
Why it's wrong here
The SA is triggered by the ACL; if the ACL is narrow, only matched traffic creates SAs.
✗
The router has a NAT rule that bypasses IPsec for HTTP traffic.
Why it's wrong here
NAT can interfere, but the question does not mention NAT.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the transform-set or the IPsec SA itself filters traffic by port or protocol, when in reality the ACL is the sole mechanism for defining interesting traffic in a crypto map.
Detailed technical explanation
How to think about this question
Under the hood, the crypto map ACL acts as a traffic selector for IPsec; when a packet matches a permit entry, the router checks for an existing IPsec SA and, if none exists, triggers IKE to negotiate one. If the ACL is too narrow (e.g., only 'permit icmp any any'), HTTP packets (TCP port 80) are not matched, so they bypass IPsec entirely. In real-world scenarios, this often happens when an engineer uses a test ACL during initial setup and forgets to expand it to cover all desired traffic, leading to partial encryption.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL for interesting traffic only permits ICMP, so only ICMP is encrypted. — The most likely explanation is that the ACL for interesting traffic only permits ICMP, so only ICMP is encrypted. In Cisco IOS, the crypto map uses the ACL to identify which traffic should be protected by IPsec. If the ACL only matches ICMP (e.g., 'permit icmp any any'), then only ICMP packets trigger IPsec encryption; all other traffic, such as HTTP, is forwarded in clear text because it does not match the ACL and is therefore not subject to IPsec processing.
What should I do if I get this 300-410 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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