- A
There is a floating static route that points to the physical interface instead of the tunnel for some subnets.
Correct because if a floating static route (with a higher administrative distance) becomes active for some traffic, those packets will be sent out the physical interface without encryption, causing encapsulation failures.
- B
The crypto map is applied to the tunnel interface instead of the physical interface.
Why wrong: Incorrect because applying the crypto map to the tunnel interface would cause all tunnel traffic to fail, not just some packets.
- C
The IPsec SA lifetime is set too high.
Why wrong: Incorrect because SA lifetime does not cause encapsulation failures.
- D
The access list in the crypto map is too restrictive and missing some subnets.
Why wrong: Incorrect because if the access list were missing subnets, those packets would not be matched and would be sent unencrypted, but the 'pkts encaps failed' counter specifically counts packets that are matched but cannot be encrypted due to routing issues.
Quick Answer
The answer is a floating static route that points to the physical interface instead of the tunnel for some subnets. This causes the IPsec VPN pkts encaps failed counter to increment slowly because packets destined for the remote LAN are routed out the physical interface, bypassing the crypto map and thus failing to be encrypted. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that a tunnel being up does not guarantee all traffic is encrypted—routing decisions happen before encryption. A common trap is assuming the tunnel state alone confirms proper encryption, but a floating route with a higher administrative distance can become active only when the tunnel route fails, yet still cause partial failures if misconfigured. Remember the memory tip: "Tunnel up, route wrong—encaps fail all day long."
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 engineer is troubleshooting an IPsec site-to-site VPN where the tunnel is up and traffic is flowing, but the engineer notices that the 'show crypto ipsec sa' output shows the 'pkts encaps failed' counter incrementing slowly over time. The tunnel remains up. 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
There is a floating static route that points to the physical interface instead of the tunnel for some subnets.
A slow but steady increase in 'pkts encaps failed' indicates that some packets that should be encrypted are not being encrypted. This is often caused by a routing issue where some traffic to the remote LAN is being routed out an interface that does not have the crypto map, bypassing encryption.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
There is a floating static route that points to the physical interface instead of the tunnel for some subnets.
Why this is correct
Correct because if a floating static route (with a higher administrative distance) becomes active for some traffic, those packets will be sent out the physical interface without encryption, causing encapsulation failures.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The crypto map is applied to the tunnel interface instead of the physical interface.
Why it's wrong here
Incorrect because applying the crypto map to the tunnel interface would cause all tunnel traffic to fail, not just some packets.
- ✗
The IPsec SA lifetime is set too high.
Why it's wrong here
Incorrect because SA lifetime does not cause encapsulation failures.
- ✗
The access list in the crypto map is too restrictive and missing some subnets.
Why it's wrong here
Incorrect because if the access list were missing subnets, those packets would not be matched and would be sent unencrypted, but the 'pkts encaps failed' counter specifically counts packets that are matched but cannot be encrypted due to routing issues.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
- →
IPsec Site-to-Site VPN — study guide chapter
Learn the concepts, then practise the questions
- →
IPsec Site-to-Site VPN practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 300-410 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Layer 3 Technologies practice questions
Practise 300-410 questions linked to Layer 3 Technologies.
EIGRP Troubleshooting practice questions
Practise 300-410 questions linked to EIGRP Troubleshooting.
OSPF Troubleshooting (v2/v3) practice questions
Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).
BGP Troubleshooting practice questions
Practise 300-410 questions linked to BGP Troubleshooting.
Route Redistribution practice questions
Practise 300-410 questions linked to Route Redistribution.
Policy-Based Routing (PBR) practice questions
Practise 300-410 questions linked to Policy-Based Routing (PBR).
VRF-Lite practice questions
Practise 300-410 questions linked to VRF-Lite.
Route Maps and Route Filtering practice questions
Practise 300-410 questions linked to Route Maps and Route Filtering.
Administrative Distance practice questions
Practise 300-410 questions linked to Administrative Distance.
Route Summarization practice questions
Practise 300-410 questions linked to Route Summarization.
Bidirectional Forwarding Detection (BFD) practice questions
Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).
VPN Technologies practice questions
Practise 300-410 questions linked to VPN Technologies.
Practice this exam
Start a free 300-410 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
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 — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: There is a floating static route that points to the physical interface instead of the tunnel for some subnets. — A slow but steady increase in 'pkts encaps failed' indicates that some packets that should be encrypted are not being encrypted. This is often caused by a routing issue where some traffic to the remote LAN is being routed out an interface that does not have the crypto map, bypassing encryption.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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 →
Last reviewed: Jun 18, 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.
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.