Question 536 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

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.”

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.

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?

Question 1hardmultiple choice

Read the full VPN explanation →

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 →

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.

For NAT to work bidirectionally, the router must translate the source IP of the server's outbound traffic to the public IP. Static NAT should handle this automatically, but if the traffic is not matching the NAT rule, it might be due to routing or interface NAT direction.

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.

✗
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
Static NAT maps one inside address to one outside address.
✗
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: 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.

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.

What to study next

Got this wrong? Here's your next step.

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.

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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 — Static NAT maps one inside address to one outside address..

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. — For NAT to work bidirectionally, the router must translate the source IP of the server's outbound traffic to the public IP. Static NAT should handle this automatically, but if the traffic is not matching the NAT rule, it might be due to routing or interface NAT direction.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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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: Static NAT translates the destination IP, but if the server's response uses a different source IP (e.g., due to multiple interfaces or routing), the return traffic may not be translated back. However, a common issue is that the server's firewall or local ACL blocks SSH, or the router's NAT is not translating the return traffic correctly.

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