- A
Configure ip nat translation timeout 60 for UDP to align with VoIP timers.
Reducing the NAT timeout for UDP ensures that stale entries are cleared quickly, preventing mismatches with VoIP session refreshes.
- B
Add ip nat inside source list 100 interface Tunnel0 overload to the spoke router.
Why wrong: This would duplicate NAT configuration and cause conflicts, not resolve the timeout issue.
- C
Change the DMVPN tunnel mode to GRE over IPsec with no NAT.
Why wrong: This may avoid NAT but is a major redesign and does not address the root cause of timeout mismatch.
- D
Use ip nat outside source list 100 interface Tunnel0 overload.
Why wrong: Outside source NAT is for translating external addresses, not relevant to inside VoIP traffic.
Question 225 of 2,152
NAT Timeout for VoIP Traffic in DMVPN
This 300-410 practice question tests your understanding of nat and pat. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 large enterprise network is experiencing intermittent connectivity failures for VoIP traffic traversing a DMVPN hub-and-spoke topology. Hub router R1 has the following relevant configuration: ip nat inside source list 100 interface Tunnel0 overload. Spoke router R2 shows: show ip nat translations: Pro Inside global Inside local Outside local Outside global --- 10.1.1.1 192.168.1.1 203.0.113.1 203.0.113.1. VoIP calls drop after 30 seconds. What is the root cause?
Quick Answer
The answer is to configure `ip nat translation timeout 60` for UDP to align with VoIP timers. This is correct because in a DMVPN hub-and-spoke topology, PAT overload on the tunnel interface creates NAT entries for VoIP traffic, but the default UDP NAT timeout of 300 seconds does not match the shorter keepalive or session refresh intervals used by VoIP protocols like SIP or RTP. When the DMVPN tunnel rekeys or the VoIP session refreshes, the stale NAT entry causes asymmetric routing and dropped packets after approximately 30 seconds. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of how NAT interacts with dynamic overlay networks and the importance of tuning application-layer timers. A common trap is assuming NAT only affects TCP or that DMVPN traffic is automatically exempt; in reality, any traffic traversing the tunnel interface with overload configured will be translated. Memory tip: think “VoIP refreshes fast, NAT timeout must match—adjust UDP to 60 seconds to keep the call path stable.”
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
✓
Configure ip nat translation timeout 60 for UDP to align with VoIP timers.
The default NAT translation timeout for UDP on Cisco routers is 300 seconds, but in a DMVPN network, the translation may be removed prematurely due to routing changes or other factors. The command 'ip nat translation timeout 60 for UDP' explicitly sets the timeout to 60 seconds, which aligns with typical VoIP keepalive intervals (every 30–60 seconds). This ensures that the NAT entry remains active as long as keepalives are received frequently enough, preventing call drops after 30 seconds of idle time.
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.
- ✓
Configure ip nat translation timeout 60 for UDP to align with VoIP timers.
- ✗
Add ip nat inside source list 100 interface Tunnel0 overload to the spoke router.
Why it's wrong here
This would duplicate NAT configuration and cause conflicts, not resolve the timeout issue.
- ✗
Change the DMVPN tunnel mode to GRE over IPsec with no NAT.
Why it's wrong here
This may avoid NAT but is a major redesign and does not address the root cause of timeout mismatch.
- ✗
Use ip nat outside source list 100 interface Tunnel0 overload.
Why it's wrong here
Outside source NAT is for translating external addresses, not relevant to inside VoIP traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that VoIP call drops are due to NAT configuration errors or tunnel mode issues, rather than the default NAT timeout values that are too short for UDP-based real-time traffic.
Detailed technical explanation
How to think about this question
The default NAT UDP timeout is 60 seconds (or 5 minutes on some IOS versions), but VoIP calls often use RTP with keepalives or silence suppression that can cause idle periods longer than 30 seconds, leading to premature NAT entry removal. The 'ip nat translation timeout' command adjusts the per-protocol timeout; for UDP, the default is 300 seconds in some versions, but the 30-second drop suggests a lower timeout or a misconfigured 'ip nat translation udp-timeout'. In real-world scenarios, VoIP providers often recommend setting NAT timeouts to at least 60 seconds to accommodate jitter buffers and re-invites.
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
What to study next
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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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure ip nat translation timeout 60 for UDP to align with VoIP timers. — The default NAT translation timeout for UDP on Cisco routers is 300 seconds, but in a DMVPN network, the translation may be removed prematurely due to routing changes or other factors. The command 'ip nat translation timeout 60 for UDP' explicitly sets the timeout to 60 seconds, which aligns with typical VoIP keepalive intervals (every 30–60 seconds). This ensures that the NAT entry remains active as long as keepalives are received frequently enough, preventing call drops after 30 seconds of idle time.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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