- A
The NAT translation table is full; clear the table with clear ip nat translation *.
Why wrong: This is incorrect because the NAT translation table is clearly not full; only one translation is present. Clearing the table would not resolve the issue.
- B
The access-list 100 is misconfigured and blocking the server's IP.
Correct. The ACL 100 is used to define which inside hosts are translated. If it does not permit the source IP 192.168.1.1, the NAT translation fails, and the debug shows 'no buffer'. Review and correct the ACL configuration.
- C
The interface GigabitEthernet0/1 is down.
Why wrong: If the interface were down, NAT would not attempt translations, and the debug would show a different error such as 'interface down'. The presence of a translation entry indicates the interface is operational.
- D
The external server is unreachable due to routing issues.
Why wrong: Routing issues would prevent reachability but would not cause a NAT translation failure with 'no buffer'.
NAT Translation Failed (No Buffer) — Root Cause and Fix | Cisco CCNP ENARSI 300-410 Explained
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.
Router R1 is performing NAT for internal users to access the internet. The configuration includes: ip nat inside source list 100 interface GigabitEthernet0/1 overload. Internal hosts cannot reach a specific external server at 203.0.113.50. Router R1 shows: show ip nat translations: Pro Inside global Inside local Outside local Outside global --- 10.1.1.1 192.168.1.1 203.0.113.50 203.0.113.50. Debug ip nat shows 'NAT: translation failed (no buffer)'. What is the root cause?
Quick Answer
The answer is that the NAT translation table is full, and the root cause is an exhaustion of available ports for PAT (Port Address Translation), which triggers the 'no buffer' error in the debug output. This occurs because the router’s NAT table has reached its maximum capacity for dynamic translations, often due to a high volume of short-lived connections that are not properly closed, leaving stale entries that consume all available inside global ports. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of NAT overload behavior and the specific meaning of the "translation failed (no buffer)" message, which is a common trap—students often misdiagnose it as a memory or interface issue rather than a table capacity problem. The immediate fix is to clear the table with `clear ip nat translation *`, but for a permanent solution, you should increase the NAT translation timeout or expand the port range. Memory tip: "No buffer" means "no room in the table"—think of a full parking lot with no empty spaces for new cars.
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 access-list 100 is misconfigured and blocking the server's IP.
The debug message 'NAT: translation failed (no buffer)' typically indicates a full NAT translation table, but the show ip nat translations output reveals only a single entry, contradicting that diagnosis. The most likely cause is that access-list 100 is misconfigured, as it is used to match inside local addresses for translation. If the ACL does not permit the inside host's source IP (192.168.1.1), the router will not create a translation and may generate a generic 'no buffer' error. Therefore, the root cause is a misconfigured ACL.
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 NAT translation table is full; clear the table with clear ip nat translation *.
Why it's wrong here
This is incorrect because the NAT translation table is clearly not full; only one translation is present. Clearing the table would not resolve the issue.
- ✓
The access-list 100 is misconfigured and blocking the server's IP.
Why this is correct
Correct. The ACL 100 is used to define which inside hosts are translated. If it does not permit the source IP 192.168.1.1, the NAT translation fails, and the debug shows 'no buffer'. Review and correct the ACL configuration.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The interface GigabitEthernet0/1 is down.
Why it's wrong here
If the interface were down, NAT would not attempt translations, and the debug would show a different error such as 'interface down'. The presence of a translation entry indicates the interface is operational.
- ✗
The external server is unreachable due to routing issues.
Why it's wrong here
Routing issues would prevent reachability but would not cause a NAT translation failure with 'no buffer'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco's 'no buffer' debug message is often associated with NAT table exhaustion, but candidates must verify the number of active translations. In this scenario, the single entry in the translation table indicates the table is not full, pointing to an ACL configuration issue instead.
Trap categories for this question
Command / output trap
If the interface were down, NAT would not attempt translations, and the debug would show a different error such as 'interface down'. The presence of a translation entry indicates the interface is operational.
Detailed technical explanation
How to think about this question
NAT translation tables have a finite size, often configurable via 'ip nat translation max-entries' (default varies by platform, e.g., 512 on Cisco IOS). When the table is full, PAT cannot create new entries even if the same source IP is used, because each unique source port or destination requires a separate entry. In real-world scenarios, this can occur during a DoS attack or when many internal hosts simultaneously initiate connections, exhausting the table before entries time out (default 24 hours for UDP/TCP).
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
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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 access-list 100 is misconfigured and blocking the server's IP. — The debug message 'NAT: translation failed (no buffer)' typically indicates a full NAT translation table, but the show ip nat translations output reveals only a single entry, contradicting that diagnosis. The most likely cause is that access-list 100 is misconfigured, as it is used to match inside local addresses for translation. If the ACL does not permit the inside host's source IP (192.168.1.1), the router will not create a translation and may generate a generic 'no buffer' error. Therefore, the root cause is a misconfigured ACL.
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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