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300-410 Practice Question: Is troubleshooting a Cisco router that is…
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?
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
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.
Visual reference
Quick reference
AAA Protocol Comparison
| Protocol | Port(s) | Encryption | Transport | Primary Use |
|---|---|---|---|---|
| RADIUS | 1812 / 1813 | Password only | UDP | Network access control |
| TACACS+ | 49 | Full packet | TCP | Device administration |
| Diameter | 3868 | Full session | TCP / SCTP | Carrier / mobile networks |
| 802.1X | — | EAP-based | Layer 2 | Port-based access control |
TACACS+ encrypts the entire packet; RADIUS only encrypts the password field — a key exam distinction.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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