Question 302 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

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.”

300-410 NAT and PAT Practice Question

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.

Question 1hardmultiple choice

Read the full NAT/PAT explanation →

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 →

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.

SIP embeds IP addresses in the payload; PAT only translates the IP header, not the application layer. The SIP server sends media to the private IP, which is unreachable. The fix is to use SIP ALG or fixup to translate the embedded addresses.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

Detailed technical explanation

How to think about this question

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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.

What to study next

Got this wrong? Here's your next step.

Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 300-410 question test?

NAT and PAT — This question tests NAT and PAT — Static NAT maps one inside address to one outside address..

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. — SIP embeds IP addresses in the payload; PAT only translates the IP header, not the application layer. The SIP server sends media to the private IP, which is unreachable. The fix is to use SIP ALG or fixup to translate the embedded addresses.

What should I do if I get this 300-410 question wrong?

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?

Static NAT maps one inside address to one outside address.

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