Question 536 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

Static NAT Asymmetric Routing — Troubleshooting Return Traffic

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.

A network engineer is troubleshooting NAT for a VPN tunnel. The router has a static NAT rule 'ip nat inside source static 10.0.0.10 203.0.113.10' for a server. The VPN traffic from the remote site to 203.0.113.10 is being NATed to 10.0.0.10, but the return traffic from the server to the remote site is not being translated back. The engineer sees that the server sends packets with source 10.0.0.10 to the remote site's public IP. What should the engineer do to fix this?

A
Add an 'ip nat outside' command on the inside interface.
Why wrong: Incorrect because the inside interface should be 'ip nat inside'; adding outside would break the NAT logic.
B
Configure a route-map to exempt the VPN traffic from NAT.
Why wrong: Incorrect because the issue is that NAT is not translating the outbound traffic; exempting would make it worse.
C
Ensure that the router has a route to the remote site's public IP via the outside interface, and that the static NAT entry is correctly applied.
Correct because if the return traffic from the server is routed out a different interface (e.g., a VPN tunnel interface), the NAT might not be applied; the router needs to route the traffic via the outside interface where NAT is configured.
D
Change the static NAT to 'ip nat inside source static 10.0.0.10 203.0.113.10 extendable'.
Why wrong: Incorrect because 'extendable' allows multiple static NATs for the same inside host, but it does not fix the routing/interface issue.

Full breakdown with real-world context →

Quick Answer

The answer is to ensure the router has a route to the remote site’s public IP via the outside interface and that the static NAT entry is correctly applied. This is because static NAT creates a one-to-one mapping that should translate return traffic automatically, but asymmetric routing breaks this when the server’s outbound packets take a different path than the inbound VPN traffic, causing the router to miss the translation. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of NAT order of operations and interface directionality—a common trap is forgetting that the router must see both directions of traffic on the same NAT inside/outside interfaces. The key is that static NAT does not require an access-list, but it does require the return traffic’s destination (the remote public IP) to be reachable via the outside interface; otherwise, the router forwards the server’s source-10.0.0.10 packet untranslated. Memory tip: “Static NAT is a two-way street—if the return route doesn’t meet the outside interface, the translation won’t meet the packet.”

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

✓

Ensure that the router has a route to the remote site's public IP via the outside interface, and that the static NAT entry is correctly applied.

The issue is that the router lacks a route back to the remote site's public IP via the outside interface, or the static NAT entry is not correctly applied for the return traffic. Without a proper route, the router cannot determine that the return packets (from the inside server 10.0.0.10 to the remote public IP) should be translated back to 203.0.113.10. Ensuring the route exists and the static NAT is correctly configured allows the router to perform the reverse translation and forward the traffic out the outside interface.

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.

✗
Add an 'ip nat outside' command on the inside interface.
Why it's wrong here
Incorrect because the inside interface should be 'ip nat inside'; adding outside would break the NAT logic.
✗
Configure a route-map to exempt the VPN traffic from NAT.
Why it's wrong here
Incorrect because the issue is that NAT is not translating the outbound traffic; exempting would make it worse.
✓
Ensure that the router has a route to the remote site's public IP via the outside interface, and that the static NAT entry is correctly applied.
Why this is correct
Correct because if the return traffic from the server is routed out a different interface (e.g., a VPN tunnel interface), the NAT might not be applied; the router needs to route the traffic via the outside interface where NAT is configured.
Related concept
Read the scenario before looking for a memorised answer.
✗
Change the static NAT to 'ip nat inside source static 10.0.0.10 203.0.113.10 extendable'.
Why it's wrong here
Incorrect because 'extendable' allows multiple static NATs for the same inside host, but it does not fix the routing/interface issue.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that NAT issues are always due to missing 'ip nat outside' or 'extendable' keywords, when the real problem is a missing route or incorrect interface assignment for the return traffic.

Detailed technical explanation

How to think about this question

Cisco IOS NAT uses a translation table that maps inside local to inside global addresses; for return traffic, the router must perform a reverse lookup in this table. The router also requires a route to the destination (remote public IP) via the outside interface to know which interface to use for the translated packet; without this route, the router may drop the packet or send it out the wrong interface. In real-world scenarios, this often occurs when the outside interface is not the default gateway for the remote network, or when a static route is missing.

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: Ensure that the router has a route to the remote site's public IP via the outside interface, and that the static NAT entry is correctly applied. — The issue is that the router lacks a route back to the remote site's public IP via the outside interface, or the static NAT entry is not correctly applied for the return traffic. Without a proper route, the router cannot determine that the return packets (from the inside server 10.0.0.10 to the remote public IP) should be translated back to 203.0.113.10. Ensuring the route exists and the static NAT is correctly configured allows the router to perform the reverse translation and forward the traffic out the outside interface.

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.

About these practice questions

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer is troubleshooting NAT for a web server that is reachable from the internet via a static NAT mapping 203.0.113.20 to 10.0.0.20. The server responds to HTTP requests, but the engineer cannot SSH to the server from the internet. 'Show ip nat translations' shows the static entry. The router's ACL on the outside interface permits TCP port 22 to 203.0.113.20. What is the most likely cause?

hard

A.The server's default gateway is not the router's inside interface.
✓ B.The router's NAT is not translating the return traffic for SSH because the server sends packets with a different source IP.
C.The SSH service is not running on the server.
D.The router's ACL is blocking SSH traffic despite the permit statement.

Why B: The static NAT entry maps 203.0.113.20 to 10.0.0.20 only for inbound traffic. When the server responds to SSH, it uses its own IP (10.0.0.20) as the source, not the mapped public IP. The router does not translate the source address of the return packet back to 203.0.113.20 because there is no outbound NAT rule for that traffic. The SSH client on the internet receives a reply from 10.0.0.20 instead of 203.0.113.20 and drops it. HTTP works because the server's HTTP response may be subject to a different NAT rule or the client is tolerant of mismatched source addresses, but typically the same issue would occur unless the server uses the public IP as its source for all traffic. The default gateway is correct since the server can send traffic out. The ACL permits SSH, so that is not the issue. Therefore, the most likely cause is that the router is not translating the return traffic for SSH because the server sends packets with a different source IP.

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