- A
uRPF strict mode on the physical interface drops packets from spokes because the reverse path to the spoke's tunnel IP is via the DMVPN tunnel interface, not the physical interface.
The hub routes traffic to spoke tunnel IPs through the tunnel interface, so the reverse path check fails on the physical interface, causing drops.
- B
uRPF strict mode on the physical interface drops packets because the source IP of the spoke is not in the routing table at all.
Why wrong: The spoke's tunnel IP is in the routing table; the issue is the interface mismatch.
- C
uRPF strict mode is incompatible with DMVPN because the tunnel interface uses GRE encapsulation, which modifies the source IP.
Why wrong: uRPF checks the source IP of the outer IP header; GRE encapsulation does not change the source IP of the encapsulated packet.
- D
The 'allow-default' option is not configured, which is required for uRPF to work with DMVPN.
Why wrong: The 'allow-default' option allows the use of a default route for the reverse path check, but the issue here is interface mismatch, not default route.
Quick Answer
The answer is that uRPF strict mode on the hub’s physical interface drops spoke-to-spoke traffic because the reverse path to the spoke’s tunnel IP exists via the DMVPN tunnel interface, not the physical WAN interface. In strict mode, uRPF verifies that the source IP of an incoming packet has a route back through the exact same interface; since the hub’s routing table points the spoke’s tunnel IP out the tunnel interface, the physical interface check fails, causing the drop. This scenario is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding of how uRPF interacts with DMVPN overlay routing—many engineers mistakenly apply strict mode on the physical interface without considering the asymmetric path. The key insight is that uRPF strict mode is interface-centric, while DMVPN traffic often uses different inbound and outbound interfaces. Remember the mnemonic: “Strict checks the same NIC, but DMVPN paths are asymmetric.”
300-410 DMVPN Practice Question
This 300-410 practice question tests your understanding of dmvpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on the DMVPN hub's physical interface facing the WAN. Unexpectedly, spokes are unable to communicate with each other via the hub, even though direct spoke-to-spoke tunnels are working. Which is the most likely explanation?
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.
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
uRPF strict mode on the physical interface drops packets from spokes because the reverse path to the spoke's tunnel IP is via the DMVPN tunnel interface, not the physical interface.
uRPF strict mode checks that the source IP address of incoming packets has a route back to the source via the same interface. In a DMVPN network, when a spoke sends traffic to another spoke via the hub, the source IP is the spoke's tunnel IP. The hub's routing table may have a route to that spoke's tunnel IP via the DMVPN tunnel interface, not the physical WAN interface. Therefore, uRPF strict mode on the physical interface drops the packet because the reverse path is not through the same interface. The fix is to use uRPF loose mode or allow-default option, or apply uRPF on the tunnel interface.
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.
- ✓
uRPF strict mode on the physical interface drops packets from spokes because the reverse path to the spoke's tunnel IP is via the DMVPN tunnel interface, not the physical interface.
Why this is correct
The hub routes traffic to spoke tunnel IPs through the tunnel interface, so the reverse path check fails on the physical interface, causing drops.
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.
- ✗
uRPF strict mode on the physical interface drops packets because the source IP of the spoke is not in the routing table at all.
Why it's wrong here
The spoke's tunnel IP is in the routing table; the issue is the interface mismatch.
- ✗
uRPF strict mode is incompatible with DMVPN because the tunnel interface uses GRE encapsulation, which modifies the source IP.
Why it's wrong here
uRPF checks the source IP of the outer IP header; GRE encapsulation does not change the source IP of the encapsulated packet.
- ✗
The 'allow-default' option is not configured, which is required for uRPF to work with DMVPN.
Why it's wrong here
The 'allow-default' option allows the use of a default route for the reverse path check, but the issue here is interface mismatch, not default route.
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?
DMVPN — This question tests DMVPN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: uRPF strict mode on the physical interface drops packets from spokes because the reverse path to the spoke's tunnel IP is via the DMVPN tunnel interface, not the physical interface. — uRPF strict mode checks that the source IP address of incoming packets has a route back to the source via the same interface. In a DMVPN network, when a spoke sends traffic to another spoke via the hub, the source IP is the spoke's tunnel IP. The hub's routing table may have a route to that spoke's tunnel IP via the DMVPN tunnel interface, not the physical WAN interface. Therefore, uRPF strict mode on the physical interface drops the packet because the reverse path is not through the same interface. The fix is to use uRPF loose mode or allow-default option, or apply uRPF on the tunnel interface.
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.
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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Last reviewed: Jun 18, 2026
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