Question 557 of 2,152

NAT and PAT →mediumMultiple ChoiceObjective-mapped

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.

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.

Full breakdown with real-world context →

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.

The high CPU utilization and thousands of NAT translations indicate that the router is maintaining an excessive number of translation entries. The default NAT timeout for TCP is 86400 seconds (24 hours) and for UDP is 300 seconds. If the timeout is set too high, old entries are not cleared promptly, causing the translation table to grow large and consuming CPU resources for lookups and maintenance. This is the most likely cause given the symptoms.

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 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
Read the scenario before looking for a memorised answer.
✗
The ACL is too permissive, allowing traffic from outside to initiate NAT.
Why it's wrong here
Incorrect because ACL 1 is used for inside source NAT; it does not affect outside-initiated traffic.
✗
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: answer the scenario, not the keyword

Cisco often tests the misconception that high CPU and many NAT entries are always due to an attack or ACL misconfiguration, when the real issue is often the NAT translation timeout being too high, causing stale entries to accumulate.

Detailed technical explanation

How to think about this question

NAT overload (PAT) uses a single public IP and multiplexes translations via source port numbers. The default timeout values are defined in RFC 4787 and can be adjusted with 'ip nat translation timeout' or per-protocol timers like 'ip nat translation udp-timeout'. In real-world scenarios, misconfigured timeouts can lead to table exhaustion, especially with many short-lived connections (e.g., web browsing), where entries linger far longer than needed, causing high CPU and memory usage.

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.

Visual reference

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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FAQ

Questions learners often ask

What does this 300-410 question test?

NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..

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. — The high CPU utilization and thousands of NAT translations indicate that the router is maintaining an excessive number of translation entries. The default NAT timeout for TCP is 86400 seconds (24 hours) and for UDP is 300 seconds. If the timeout is set too high, old entries are not cleared promptly, causing the translation table to grow large and consuming CPU resources for lookups and maintenance. This is the most likely cause given the symptoms.

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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