The answer is the 'no-proxy-arp' command, which prevents the ASA from responding to ARP requests for the public IP 203.0.113.10, blocking external access. When proxy ARP is disabled, the ASA will not reply to upstream routers’ ARP queries for that address, so traffic destined for the web server never reaches the ASA for NAT processing—even though the NAT rule itself is correctly configured. On the CISA exam, this scenario tests your understanding of how Layer 2 ARP behavior interacts with Layer 3 NAT on firewalls; a common trap is assuming a missing NAT rule is the culprit when the real issue is the ASA’s failure to intercept traffic at the data link layer. Remember the mnemonic: “No proxy, no ARP reply—traffic passes by.”
CISA Practice Question: Information Systems Acquisition, Development and Implementation
This CISA practice question tests your understanding of information systems acquisition, development and implementation. 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.
Exhibit
Refer to the exhibit.
```
! Cisco ASA configuration snippet
access-list OUTSIDE_IN extended permit tcp any host 203.0.113.10 eq www
access-list OUTSIDE_IN extended permit tcp any host 203.0.113.10 eq https
access-list OUTSIDE_IN extended deny ip any any log
!
object network WEB_SERVER
host 203.0.113.10
nat (inside,outside) source static any any destination static WEB_SERVER WEB_SERVER no-proxy-arp route-lookup
!
```
Refer to the exhibit. A security administrator is troubleshooting why external users cannot reach the web server at 203.0.113.10 from the internet. Based on the configuration, what is the MOST likely issue?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The 'no-proxy-arp' option prevents the ASA from responding to ARP requests for the public IP
The 'no-proxy-arp' command disables proxy ARP on the ASA interface for the public IP address 203.0.113.10. Without proxy ARP, the ASA will not respond to ARP requests from upstream routers for that IP, so traffic destined to the web server is never delivered to the ASA for NAT processing. This is the most likely cause because the NAT rule exists but the ASA cannot intercept the traffic at Layer 2.
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.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a NAT rule alone is sufficient for inbound traffic, overlooking the Layer 2 requirement that the ASA must respond to ARP for the public IP via proxy ARP.
Detailed technical explanation
How to think about this question
Proxy ARP is a Cisco ASA feature that allows the device to answer ARP requests for a mapped public IP address, even though that IP is not configured on its interface. When 'no-proxy-arp' is set, the ASA silently drops ARP requests for that IP, causing upstream routers to believe the IP is unreachable. This is a common misconfiguration when using static NAT with a public IP that is not directly assigned to the ASA interface, as the ASA must proxy for that IP to attract 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.
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.
Information Systems Acquisition, Development and Implementation — This question tests Information Systems Acquisition, Development and Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The 'no-proxy-arp' option prevents the ASA from responding to ARP requests for the public IP — The 'no-proxy-arp' command disables proxy ARP on the ASA interface for the public IP address 203.0.113.10. Without proxy ARP, the ASA will not respond to ARP requests from upstream routers for that IP, so traffic destined to the web server is never delivered to the ASA for NAT processing. This is the most likely cause because the NAT rule exists but the ASA cannot intercept the traffic at Layer 2.
What should I do if I get this CISA 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.
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Question Discussion
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