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.”
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 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?
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 issue is that PAT overload on the DMVPN tunnel interface causes NAT entries to be created for VoIP traffic, but the default NAT timeout (24 hours for TCP, 300 seconds for UDP) does not match the DMVPN tunnel keepalive or VoIP session timers. When the tunnel rekeys or the VoIP session refreshes, the NAT entry may be stale, causing asymmetric routing or dropped packets. The correct fix is to adjust NAT timeouts or use route-map to exempt VoIP traffic from NAT.
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.
✓
Configure ip nat translation timeout 60 for UDP to align with VoIP timers.
Why this is correct
Reducing the NAT timeout for UDP ensures that stale entries are cleared quickly, preventing mismatches with VoIP session refreshes.
Related concept
Static NAT maps one inside address to one outside address.
✗
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: 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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: Configure ip nat translation timeout 60 for UDP to align with VoIP timers. — The issue is that PAT overload on the DMVPN tunnel interface causes NAT entries to be created for VoIP traffic, but the default NAT timeout (24 hours for TCP, 300 seconds for UDP) does not match the DMVPN tunnel keepalive or VoIP session timers. When the tunnel rekeys or the VoIP session refreshes, the NAT entry may be stale, causing asymmetric routing or dropped packets. The correct fix is to adjust NAT timeouts or use route-map to exempt VoIP traffic from NAT.
What should I do if I get this 300-410 question wrong?
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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