The answer is that traffic taking multiple paths, causing packets to arrive out of order, is the most likely cause of an IPsec anti-replay check failure. This occurs because IPsec anti-replay relies on a sliding window of sequence numbers; when packets arrive from different routes or due to load balancing, their sequence numbers may fall outside or behind the current window, triggering a drop. On the ENCOR 350-401 exam, this scenario tests your understanding of how IPsec security associations (SAs) interact with network topology—specifically, that anti-replay is a stateful mechanism that assumes in-order delivery, making it vulnerable to path asymmetry. A common trap is to suspect encryption or authentication mismatches, but the key clue is the intermittent nature of the drops and the increasing anti-replay counter. Memory tip: think “multiple paths, multiple problems”—if your traffic splits, your anti-replay window splits too.
CCNP WAN Technologies Practice Question
This 350-401 practice question tests your understanding of wan technologies. 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 a site-to-site VPN that uses IPsec with IKEv1. The tunnel is established, but traffic is intermittently dropped. The engineer checks the 'show crypto ipsec sa' output and sees that the number of packets that failed anti-replay check is increasing. What is the most likely cause of this issue?
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.
The IPsec SA is using a weak encryption algorithm.
Why wrong: Incorrect. The encryption algorithm does not affect anti-replay checks.
B
The IPsec SA is using ESP in tunnel mode with authentication only.
Why wrong: Incorrect. The mode or authentication does not cause anti-replay failures.
C
The traffic is taking multiple paths, causing packets to arrive out of order.
Correct. Anti-replay checks rely on sequence numbers. If packets arrive out of order, the receiver may drop them if they fall outside the anti-replay window.
D
The IPsec SA lifetime is too short, causing frequent rekeying.
Why wrong: Incorrect. Frequent rekeying can cause temporary drops, but it would not cause anti-replay failures specifically.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The traffic is taking multiple paths, causing packets to arrive out of order.
Anti-replay is a security feature in IPsec that uses sequence numbers to prevent replay attacks. If packets arrive out of order (e.g., due to different paths or latency), the anti-replay window may drop them. This is common when there are multiple paths or when the IPsec SA is used for traffic that is load-balanced across different links.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 IPsec SA is using a weak encryption algorithm.
Why it's wrong here
Incorrect. The encryption algorithm does not affect anti-replay checks.
✗
The IPsec SA is using ESP in tunnel mode with authentication only.
Why it's wrong here
Incorrect. The mode or authentication does not cause anti-replay failures.
✓
The traffic is taking multiple paths, causing packets to arrive out of order.
Why this is correct
Correct. Anti-replay checks rely on sequence numbers. If packets arrive out of order, the receiver may drop them if they fall outside the anti-replay window.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The IPsec SA lifetime is too short, causing frequent rekeying.
Why it's wrong here
Incorrect. Frequent rekeying can cause temporary drops, but it would not cause anti-replay failures specifically.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
WAN Technologies — This question tests WAN Technologies — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The traffic is taking multiple paths, causing packets to arrive out of order. — Anti-replay is a security feature in IPsec that uses sequence numbers to prevent replay attacks. If packets arrive out of order (e.g., due to different paths or latency), the anti-replay window may drop them. This is common when there are multiple paths or when the IPsec SA is used for traffic that is load-balanced across different links.
What should I do if I get this 350-401 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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