- A
The application uses protocols that do not have port numbers, such as GRE, and PAT cannot handle them.
Correct. PAT requires port numbers; non-TCP/UDP protocols fail.
- B
The NAT pool is exhausted.
Why wrong: Exhaustion would affect all traffic, not just non-standard ports.
- C
The inside interface is not configured correctly.
Why wrong: Interface misconfiguration would affect all traffic.
- D
The outside interface has a different MTU.
Why wrong: MTU mismatch would cause fragmentation issues, not port-specific failure.
Question 1,017 of 2,152
PAT Limitations — Non-TCP/UDP Protocols (GRE, ESP) | 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.
An engineer configures NAT overload (PAT) on a router to translate internal addresses to a single public IP. Users can browse the web, but some applications that use non-standard ports fail. Which is the most likely explanation?
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.
Quick Answer
The most likely explanation is that the application uses protocols without port numbers, such as GRE or IPsec ESP, which PAT cannot handle. PAT, or Port Address Translation, relies on unique Layer 4 port numbers to map multiple internal addresses to a single public IP, but protocols like GRE (protocol 47) and ESP (protocol 50) operate directly over IP and lack any port concept, making translation impossible. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of NAT overload limitations beyond standard TCP/UDP traffic—a common trap is assuming PAT works for all IP protocols, when in fact it only supports those with port numbers. Remember the memory tip: “No port, no PAT—GRE and ESP are the culprits.”
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 application uses protocols that do not have port numbers, such as GRE, and PAT cannot handle them.
PAT (Port Address Translation) relies on TCP/UDP port numbers to multiplex multiple internal addresses to a single public IP. Protocols like GRE (Generic Routing Encapsulation) and IPsec ESP/AH do not use port numbers, so PAT cannot differentiate between multiple sessions using these protocols. This causes the translation to fail for such applications, even though standard web traffic (HTTP/HTTPS) works fine.
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 application uses protocols that do not have port numbers, such as GRE, and PAT cannot handle them.
- ✗
The NAT pool is exhausted.
Why it's wrong here
Exhaustion would affect all traffic, not just non-standard ports.
- ✗
The inside interface is not configured correctly.
Why it's wrong here
Interface misconfiguration would affect all traffic.
- ✗
The outside interface has a different MTU.
Why it's wrong here
MTU mismatch would cause fragmentation issues, not port-specific failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that PAT works for all IP traffic, when in fact it only supports TCP, UDP, and ICMP (with limitations) because it requires port numbers for multiplexing.
Detailed technical explanation
How to think about this question
PAT creates a translation entry mapping {source IP, source port} to {public IP, unique port}. For protocols like GRE (IP protocol 47) or IPsec ESP (IP protocol 50), there is no port field in the transport layer, so PAT cannot create a unique mapping for multiple internal hosts. Cisco IOS uses 'ip nat outside source static' or route maps with 'match protocol' to handle such cases, but by default PAT fails for non-TCP/UDP traffic.
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
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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 application uses protocols that do not have port numbers, such as GRE, and PAT cannot handle them. — PAT (Port Address Translation) relies on TCP/UDP port numbers to multiplex multiple internal addresses to a single public IP. Protocols like GRE (Generic Routing Encapsulation) and IPsec ESP/AH do not use port numbers, so PAT cannot differentiate between multiple sessions using these protocols. This causes the translation to fail for such applications, even though standard web traffic (HTTP/HTTPS) works fine.
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.
About these practice questions
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Last reviewed: Jul 4, 2026
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