- A
SYN
Why wrong: SYN starts a connection; it does not trigger the shorter timeout.
- B
FIN or RST
FIN and RST indicate connection termination, causing the NAT entry to expire quickly.
- C
ACK
Why wrong: ACK is part of normal data transfer and does not trigger the shorter timeout.
- D
URG
Why wrong: URG is urgent data and does not affect NAT timeout.
Question 1,500 of 2,152
TCP Flags Influencing NAT Translation Timeout
This 300-410 practice question tests your understanding of nat and pat. 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.
Which TCP flag combination triggers the NAT translation timeout to change from the default to the 'ip nat translation tcp-timeout' value?
Quick Answer
The answer is the FIN or RST flag. When a TCP connection terminates normally with a FIN or is abruptly reset with an RST, the NAT router recognizes the session is closing and switches the translation timeout from the default idle timer to the shorter value configured under 'ip nat translation tcp-timeout', which defaults to 60 seconds. This behavior is critical for efficient NAT table management, preventing stale entries from lingering for the full default timeout (typically 24 hours for TCP). On the Cisco CCNP ENARSI 300-410 exam, this concept often appears in troubleshooting scenarios where NAT translations are not clearing quickly enough, or in questions testing your understanding of how TCP flags influence stateful inspection. A common trap is assuming the SYN or ACK flags trigger the timeout change—they do not; only the connection-closing flags matter. Memory tip: think "Finish or Reset" to recall that FIN and RST are the flags that trigger the shorter timeout.
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
✓
FIN or RST
By default, NAT translations for TCP sessions use a timeout based on the TCP connection's state. When a TCP session ends with a FIN or RST flag, the NAT device immediately transitions the translation to a 'finish' or 'reset' state, which then uses the value configured under 'ip nat translation tcp-timeout' (default 60 seconds) instead of the longer default idle timeout (typically 86400 seconds). This allows the NAT table to reclaim resources quickly after a connection is explicitly terminated.
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.
- ✗
SYN
Why it's wrong here
SYN starts a connection; it does not trigger the shorter timeout.
- ✓
FIN or RST
Why this is correct
FIN and RST indicate connection termination, causing the NAT entry to expire quickly.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
ACK
Why it's wrong here
ACK is part of normal data transfer and does not trigger the shorter timeout.
- ✗
URG
Why it's wrong here
URG is urgent data and does not affect NAT timeout.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that any TCP flag (like SYN or ACK) can trigger the shorter timeout, but only FIN or RST explicitly signal connection termination and thus change the NAT timeout to the configured tcp-timeout value.
Detailed technical explanation
How to think about this question
Under the hood, Cisco IOS NAT maintains per-translation state machines. When a TCP segment with FIN or RST is seen, the NAT engine marks the translation as 'timed out' and applies the configured tcp-timeout (default 60 seconds) rather than the default idle timeout. This behavior is critical in high-traffic environments (e.g., web servers) where thousands of short-lived TCP connections per second must be cleaned up quickly to avoid exhausting the NAT table. The 'show ip nat translations verbose' command reveals the timeout state for each entry.
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.
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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: FIN or RST — By default, NAT translations for TCP sessions use a timeout based on the TCP connection's state. When a TCP session ends with a FIN or RST flag, the NAT device immediately transitions the translation to a 'finish' or 'reset' state, which then uses the value configured under 'ip nat translation tcp-timeout' (default 60 seconds) instead of the longer default idle timeout (typically 86400 seconds). This allows the NAT table to reclaim resources quickly after a connection is explicitly terminated.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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