- A
The CoPP policy uses the 'class class-default' with a police action that drops traffic exceeding a low rate, and IKE/ESP packets are being classified into class-default because the ACL does not match them correctly.
If the ACL for the IKE/ESP class is misconfigured (e.g., wrong port or protocol), the packets fall into class-default, which may have a police or drop action, causing tunnel establishment to fail.
- B
The CoPP policy is applied to the wrong interface; it should be applied to the tunnel interface.
Why wrong: CoPP is applied globally to the control plane, not to interfaces; it protects the router's CPU.
- C
The IPsec tunnels use IKEv2, which uses UDP port 4500, and the CoPP policy only permits UDP 500.
Why wrong: IKEv2 uses UDP 500 and 4500; if only UDP 500 is permitted, tunnels using NAT-T would fail, but the question states some tunnels work.
- D
The CoPP policy rate-limits traffic in bps, but IKE/ESP traffic is bursty, causing drops during the initial exchange.
Why wrong: CoPP can be configured in pps or bps; bursty traffic could cause drops, but the more likely edge case is misclassification.
Quick Answer
The answer is that the CoPP ACL fails to match IKE and ESP packets correctly, causing them to fall into the class-default with a restrictive police rate. This is the most likely cause because Control Plane Policing applies a QoS policy to the router’s control plane, and any traffic not explicitly matched by a class-map ends up in the default class. If that default class has a police action that drops traffic exceeding a low rate, critical IKE (UDP 500) and ESP (protocol 50) packets get dropped, preventing IPsec tunnel establishment. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that CoPP can break IPsec tunnels even when you think you’ve permitted the right protocols—the trap is forgetting that class-default’s implicit match-all behavior can silently drop control-plane traffic. A common memory tip: “If it’s not matched, it’s default-dropped—check your ACL order and class-default police rate.”
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 Control Plane Policing (CoPP) on a router that terminates multiple IPsec site-to-site VPN tunnels. After applying the CoPP policy, some IPsec tunnels fail to establish, while others work fine. The engineer verifies that the CoPP policy permits IKE (UDP 500) and ESP (protocol 50) traffic. What is the most likely cause of the failure?
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
The CoPP policy uses the 'class class-default' with a police action that drops traffic exceeding a low rate, and IKE/ESP packets are being classified into class-default because the ACL does not match them correctly.
The most likely cause is that the CoPP ACL does not correctly match IKE (UDP 500) and ESP (protocol 50) packets, causing them to fall into the 'class-default' class. If the 'class-default' has a police action that drops traffic exceeding a low rate, these critical control-plane packets are dropped, preventing IPsec tunnel establishment. This explains why some tunnels work (those that happen to generate less traffic or are not rate-limited) while others fail.
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 CoPP policy uses the 'class class-default' with a police action that drops traffic exceeding a low rate, and IKE/ESP packets are being classified into class-default because the ACL does not match them correctly.
Why this is correct
If the ACL for the IKE/ESP class is misconfigured (e.g., wrong port or protocol), the packets fall into class-default, which may have a police or drop action, causing tunnel establishment to fail.
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 CoPP policy is applied to the wrong interface; it should be applied to the tunnel interface.
Why it's wrong here
CoPP is applied globally to the control plane, not to interfaces; it protects the router's CPU.
- ✗
The IPsec tunnels use IKEv2, which uses UDP port 4500, and the CoPP policy only permits UDP 500.
- ✗
The CoPP policy rate-limits traffic in bps, but IKE/ESP traffic is bursty, causing drops during the initial exchange.
Why it's wrong here
CoPP can be configured in pps or bps; bursty traffic could cause drops, but the more likely edge case is misclassification.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that CoPP misclassification into class-default with a restrictive police action is a common cause of partial IPsec tunnel failures, leading candidates to overlook ACL matching errors and instead focus on interface application or protocol versions.
Detailed technical explanation
How to think about this question
CoPP uses a control-plane service policy that classifies traffic into classes via ACLs or class-maps. If the ACL for IKE/ESP is misconfigured (e.g., wrong protocol number, missing UDP port, or incorrect direction), packets fall into 'class-default', which often has a restrictive police action. In real-world scenarios, this misclassification is common when engineers forget to match ESP as protocol 50 or use an ACL that only matches UDP 500 without considering NAT-T (UDP 4500) or IPsec AH (protocol 51).
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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IPsec Site-to-Site VPN — study guide chapter
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IPsec Site-to-Site VPN practice questions
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The CoPP policy uses the 'class class-default' with a police action that drops traffic exceeding a low rate, and IKE/ESP packets are being classified into class-default because the ACL does not match them correctly. — The most likely cause is that the CoPP ACL does not correctly match IKE (UDP 500) and ESP (protocol 50) packets, causing them to fall into the 'class-default' class. If the 'class-default' has a police action that drops traffic exceeding a low rate, these critical control-plane packets are dropped, preventing IPsec tunnel establishment. This explains why some tunnels work (those that happen to generate less traffic or are not rate-limited) while others fail.
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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Last reviewed: Jun 24, 2026
This 300-410 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 300-410 exam.
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