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300-410 Practice Question: Is troubleshooting NAT for a VoIP phone that uses…
A network engineer is troubleshooting NAT for a VoIP phone that uses SIP. The phone is at 192.168.2.10, and the router performs PAT to the outside interface 198.51.100.1. The phone can register with the SIP server, but calls fail after 30 seconds. The engineer notices that the SIP signaling includes the phone's private IP in the SDP body. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between control plane (SIP signaling) and data plane (RTP media) failures, and the trap here is that candidates assume registration success means NAT is working correctly, overlooking the need for ALG to translate embedded IPs in the SDP body.
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
✓
The router's SIP ALG is disabled, so the private IP in the SDP is not translated.
The SIP signaling includes the phone's private IP in the SDP body, which is used for media negotiation. When the router's SIP Application Layer Gateway (ALG) is disabled, it does not inspect and translate the private IP addresses embedded in the SDP payload. As a result, the SIP server sends media (RTP) to the private IP 192.168.2.10, which is unreachable from outside, causing the call to fail after the initial registration and signaling succeed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The PAT port range is exhausted.
Why it's wrong here
Incorrect because calls fail after 30 seconds, not immediately; port exhaustion would prevent new calls, not drop established ones.
- ✓
The router's SIP ALG is disabled, so the private IP in the SDP is not translated.
Why this is correct
Correct because without SIP ALG, the router does not inspect and translate the IP addresses inside the SIP messages, causing media to be sent to the private IP.
- ✗
The phone's default gateway is misconfigured.
Why it's wrong here
Incorrect because the phone can register, so it can reach the server; the issue is with media path, not initial connectivity.
- ✗
The outside interface has a firewall blocking UDP ports.
Why it's wrong here
Incorrect because if a firewall were blocking, the call would not even establish; the issue is specific to NAT and SIP payload.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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