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.
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.
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?
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 table is full; clear the table with clear ip nat translation *.
The 'no buffer' error indicates that the NAT translation table is full, typically due to an exhaustion of available ports for PAT. This can happen if many connections are initiated but not closed, or if the NAT pool is too small. The correct fix is to increase the number of available ports or clear stale entries.
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 NAT translation table is full; clear the table with clear ip nat translation *.
Why this is correct
Clearing the table frees up entries, but the root cause may be a need for more ports or timeout adjustments.
Related concept
Static NAT maps one inside address to one outside address.
✗
The access-list 100 is misconfigured and blocking the server's IP.
Why it's wrong here
The debug shows translation failure due to no buffer, not ACL deny.
✗
The interface GigabitEthernet0/1 is down.
Why it's wrong here
If the interface were down, no translations would be attempted at all.
✗
The external server is unreachable due to routing issues.
Why it's wrong here
The debug indicates a NAT-specific failure, not a routing problem.
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.
Trap categories for this question
Command / output trap
The debug shows translation failure due to no buffer, not ACL deny.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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 table is full; clear the table with clear ip nat translation *. — The 'no buffer' error indicates that the NAT translation table is full, typically due to an exhaustion of available ports for PAT. This can happen if many connections are initiated but not closed, or if the NAT pool is too small. The correct fix is to increase the number of available ports or clear stale entries.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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