Question 302 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

SIP ALG and NAT — Troubleshooting VoIP Call Drops

This 300-410 practice question tests your understanding of nat and pat. 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 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?

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.

A
The PAT port range is exhausted.
Why wrong: Incorrect because calls fail after 30 seconds, not immediately; port exhaustion would prevent new calls, not drop established ones.
B
The router's SIP ALG is disabled, so the private IP in the SDP is not translated.
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.
C
The phone's default gateway is misconfigured.
Why wrong: Incorrect because the phone can register, so it can reach the server; the issue is with media path, not initial connectivity.
D
The outside interface has a firewall blocking UDP ports.
Why wrong: Incorrect because if a firewall were blocking, the call would not even establish; the issue is specific to NAT and SIP payload.

Full breakdown with real-world context →

Quick Answer

The answer is that the router’s SIP ALG is disabled, so the private IP in the SDP body is not translated. SIP embeds IP addresses and port information inside the application-layer payload for media negotiation, while standard PAT only modifies the IP header. When the SIP server receives the SDP with the private address 192.168.2.10, it attempts to send media directly to that unreachable internal IP, causing call drops after the initial registration succeeds. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding of NAT application-level gateways and the distinction between header translation and payload translation—a common trap is assuming PAT alone handles all traffic. Remember: SIP ALG is the fix for embedded addresses; without it, the private IP leaks into the SDP, and the call fails when media starts. A useful memory tip is “SDP leaks, ALG tweaks.”

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.

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.

✗
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.
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 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.

Common exam traps

Common exam trap: answer the scenario, not the keyword

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.

Detailed technical explanation

How to think about this question

SIP ALG performs deep packet inspection (DPI) on SIP messages, translating private IPs and ports in the SDP (Session Description Protocol) body to the public IP and dynamically allocated PAT ports. Without ALG, the SIP server receives the private IP in the SDP and attempts to send RTP media directly to 192.168.2.10, which fails because that address is not routable over the public internet. This is a common issue with VoIP deployments where NAT traversal mechanisms like STUN or ALG are required.

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.

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

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.

Related 300-410 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Layer 3 Technologies practice questions

Practise 300-410 questions linked to Layer 3 Technologies.

EIGRP Troubleshooting practice questions

Practise 300-410 questions linked to EIGRP Troubleshooting.

OSPF Troubleshooting (v2/v3) practice questions

Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).

BGP Troubleshooting practice questions

Practise 300-410 questions linked to BGP Troubleshooting.

Route Redistribution practice questions

Practise 300-410 questions linked to Route Redistribution.

Policy-Based Routing (PBR) practice questions

Practise 300-410 questions linked to Policy-Based Routing (PBR).

VRF-Lite practice questions

Practise 300-410 questions linked to VRF-Lite.

Route Maps and Route Filtering practice questions

Practise 300-410 questions linked to Route Maps and Route Filtering.

Administrative Distance practice questions

Practise 300-410 questions linked to Administrative Distance.

Route Summarization practice questions

Practise 300-410 questions linked to Route Summarization.

Bidirectional Forwarding Detection (BFD) practice questions

Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).

VPN Technologies practice questions

Practise 300-410 questions linked to VPN Technologies.

Practice this exam

Start a free 300-410 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

300-410 practice-test guide →Study guide →Browse all practice tests

FAQ

Questions learners often ask

What does this 300-410 question test?

NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: 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.

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.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.