The answer is that a firewall or ACL blocking UDP port 1812 between the router and the RADIUS server is the most likely cause. This is correct because RADIUS authentication relies exclusively on UDP port 1812 for sending Access-Request packets; when the router can successfully ping the server, IP-level connectivity is confirmed, but the debug output showing no response indicates the RADIUS packets are being dropped at the transport layer. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to differentiate between Layer 3 reachability and Layer 4 protocol-specific filtering—a common trap is assuming a successful ping guarantees full service connectivity. Remember that RADIUS uses UDP, not TCP, so standard ICMP echo requests (ping) can pass through access-lists that explicitly deny UDP 1812. Memory tip: "Ping works, RADIUS fails? Check UDP 1812—the port that authenticates."
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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 a Cisco router that is configured for RADIUS authentication. The engineer issues 'debug radius authentication' and sees that the RADIUS server is not responding. The router can ping the RADIUS server. What is the most likely cause?
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
✓
UDP port 1812 is blocked between the router and the RADIUS server.
The RADIUS protocol uses UDP port 1812 for authentication. Since the router can ping the RADIUS server, network-layer connectivity exists, but the lack of response in the debug output indicates that the UDP packets are not reaching the server. A firewall or ACL blocking UDP 1812 between the router and the server is the most likely cause, as it prevents the RADIUS request from being received while ICMP (ping) traffic is permitted.
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.
✓
UDP port 1812 is blocked between the router and the RADIUS server.
Why this is correct
Correct because RADIUS authentication uses UDP port 1812; if blocked, the server will not receive or respond to requests.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The RADIUS server shared key is incorrect.
Why it's wrong here
Incorrect because if the shared key were incorrect, the server would respond with a reject, not a timeout.
✗
The router's IP address is not in the RADIUS server's client list.
Why it's wrong here
Incorrect because the server would still respond with a reject if the client is not authorized.
✗
The RADIUS server is down.
Why it's wrong here
Incorrect because the router can ping the server, indicating it is reachable.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between network-layer reachability (ping) and application-layer reachability (UDP port), leading candidates to incorrectly assume that a successful ping means the RADIUS server is fully operational and reachable.
Detailed technical explanation
How to think about this question
RADIUS authentication uses UDP, which is connectionless and does not provide delivery guarantees. When a firewall blocks UDP port 1812, the router sends the Access-Request packet but never receives a reply, causing the 'server not responding' debug message. In contrast, an incorrect shared key or missing client entry would still allow the packet to reach the server, which would then either send an Access-Reject or silently discard the packet, both of which would generate a different debug output (e.g., 'RADIUS: received from server' or 'RADIUS: no response from server' with a timeout).
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: UDP port 1812 is blocked between the router and the RADIUS server. — The RADIUS protocol uses UDP port 1812 for authentication. Since the router can ping the RADIUS server, network-layer connectivity exists, but the lack of response in the debug output indicates that the UDP packets are not reaching the server. A firewall or ACL blocking UDP 1812 between the router and the server is the most likely cause, as it prevents the RADIUS request from being received while ICMP (ping) traffic is permitted.
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.
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