The answer is that TCP port 49 is blocked between the router and the TACACS+ server. This is correct because the `test aaa group tacacs+ admin cisco123 new-code` command forces the router to use the TACACS+ protocol, which exclusively communicates over TCP port 49, unlike the older RADIUS or less secure methods. Since the ping succeeds, Layer 3 connectivity is intact, but the authentication failure with the `new-code` flag directly points to a firewall or ACL blocking that specific port. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to distinguish between network-layer reachability and application-layer protocol restrictions—a common trap where engineers assume a successful ping means all services are open. Remember, TACACS+ is TCP-based, so a blocked port 49 kills authentication even when ICMP works. Memory tip: “Ping works, auth fails? Check port 49—it’s the TACACS+ lifeline.”
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 TACACS+ authentication. The engineer issues 'test aaa group tacacs+ admin cisco123 new-code' and receives 'FAILED'. The router can ping the TACACS+ 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
✓
TCP port 49 is blocked between the router and the TACACS+ server.
The 'test aaa group tacacs+ admin cisco123 new-code' command uses the 'new-code' flag, which forces the test to use the TACACS+ protocol (TCP port 49) rather than the older, less secure method. Since the router can ping the TACACS+ server, Layer 3 connectivity is fine, but a failed authentication with 'new-code' strongly indicates that TCP port 49 is blocked by a firewall or ACL between the router and the server. TACACS+ relies on TCP port 49 for all communication, so blocking this port causes immediate failure.
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.
✓
TCP port 49 is blocked between the router and the TACACS+ server.
Why this is correct
Correct because TACACS+ uses TCP port 49; if blocked, the router cannot establish a connection to the server.
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 TACACS+ 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 failure due to timeout.
✗
The username 'admin' does not exist on the TACACS+ server.
Why it's wrong here
Incorrect because the test command would still receive a response from the server indicating authentication failure.
✗
The TACACS+ server is not configured for the router's IP address.
Why it's wrong here
Incorrect because the server would still respond with a reject if the client is not authorized.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between Layer 3 reachability (ping) and Layer 4 connectivity (TCP port 49), trapping candidates who assume that successful ping implies full connectivity to the TACACS+ server.
Trap categories for this question
Command / output trap
Incorrect because the test command would still receive a response from the server indicating authentication failure.
Detailed technical explanation
How to think about this question
TACACS+ uses TCP port 49 for all authentication, authorization, and accounting (AAA) traffic, and the 'new-code' flag in the test command explicitly uses the TACACS+ protocol (as opposed to the older 'old-code' which used a different packet format). Under the hood, the router sends a TACACS+ START packet to the server; if TCP port 49 is blocked, the TCP handshake fails, and the router immediately returns 'FAILED' without any server interaction. In real-world scenarios, administrators often misconfigure firewall rules to allow ICMP (ping) but block TCP port 49, leading to this exact symptom.
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: TCP port 49 is blocked between the router and the TACACS+ server. — The 'test aaa group tacacs+ admin cisco123 new-code' command uses the 'new-code' flag, which forces the test to use the TACACS+ protocol (TCP port 49) rather than the older, less secure method. Since the router can ping the TACACS+ server, Layer 3 connectivity is fine, but a failed authentication with 'new-code' strongly indicates that TCP port 49 is blocked by a firewall or ACL between the router and the server. TACACS+ relies on TCP port 49 for all communication, so blocking this port causes immediate failure.
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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