- A
The router is under a DDoS attack generating many NAT translations.
Why wrong: Incorrect while possible, it is not the most likely cause; the engineer should first check NAT timeouts.
- B
The NAT translation timeout is set too high, causing old entries to remain.
Correct because if the timeout (e.g., 'ip nat translation timeout') is high, entries for short-lived flows (like DNS) stay longer, accumulating and consuming CPU.
- C
The ACL is too permissive, allowing traffic from outside to initiate NAT.
Why wrong: Incorrect because ACL 1 is used for inside source NAT; it does not affect outside-initiated traffic.
- D
The outside interface is using a different IP than configured.
Why wrong: Incorrect because if the IP were wrong, NAT would fail entirely; the issue is performance, not connectivity.
300-410 NAT and PAT Practice Question
This 300-410 practice question tests your understanding of nat and pat. 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 configures NAT overload on a router. The inside network uses 172.16.0.0/16, and the outside interface is 198.51.100.1. The engineer uses 'ip nat inside source list 1 interface GigabitEthernet0/0 overload'. ACL 1 permits 172.16.0.0 0.0.255.255. Traffic works, but the engineer notices that the router's CPU utilization is high, and 'show ip nat translations' shows thousands of entries. 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
The NAT translation timeout is set too high, causing old entries to remain.
High CPU and many NAT entries could indicate a DoS attack or misconfiguration causing many short-lived sessions. However, a common issue is that the NAT timeout is too long, causing stale entries to accumulate.
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 router is under a DDoS attack generating many NAT translations.
Why it's wrong here
Incorrect while possible, it is not the most likely cause; the engineer should first check NAT timeouts.
- ✓
The NAT translation timeout is set too high, causing old entries to remain.
Why this is correct
Correct because if the timeout (e.g., 'ip nat translation timeout') is high, entries for short-lived flows (like DNS) stay longer, accumulating and consuming CPU.
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 ACL is too permissive, allowing traffic from outside to initiate NAT.
- ✗
The outside interface is using a different IP than configured.
Why it's wrong here
Incorrect because if the IP were wrong, NAT would fail entirely; the issue is performance, not connectivity.
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 300-410 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 300-410 question test?
NAT and PAT — This question tests NAT and PAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The NAT translation timeout is set too high, causing old entries to remain. — High CPU and many NAT entries could indicate a DoS attack or misconfiguration causing many short-lived sessions. However, a common issue is that the NAT timeout is too long, causing stale entries to accumulate.
What should I do if I get this 300-410 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 300-410 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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Last reviewed: Jun 19, 2026
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