The answer is that uRPF strict mode drops traffic from the hub because it requires the reverse path to the source to exit via the same interface the packet arrived on, and in a DMVPN spoke with multiple tunnels, asymmetric routing often violates this check. When the spoke receives a packet from the hub on its tunnel interface, uRPF strict mode performs a route lookup for the hub’s source IP; if the best reverse path points out a different interface—such as the physical WAN interface—the packet is discarded, even though the hub is reachable. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how uRPF interacts with DMVPN’s dynamic overlay topology, where the hub’s source address may be learned via a separate routing process. A common trap is assuming reachability alone satisfies uRPF, but strict mode demands interface symmetry. Remember: strict equals same interface, loose equals any interface—if your DMVPN drops, check the reverse path’s exit.
300-410 EIGRP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of eigrp troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 enables uRPF (unicast Reverse Path Forwarding) in strict mode on an interface connected to a DMVPN spoke. The spoke has multiple tunnels and receives traffic from the hub with a source IP that is not the best reverse path. Unexpectedly, the spoke drops all traffic from the hub, even though the hub is reachable via the tunnel. 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: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
uRPF strict mode requires the reverse path to be via the same interface; asymmetric routing causes drops.
uRPF strict mode checks that the source IP of incoming packets has a route back to the source via the same interface. In DMVPN, the hub's source IP may be reachable via a different interface (e.g., physical interface) than the tunnel interface where the packet arrives. This asymmetric routing causes uRPF strict mode to drop the packets. The solution is to use loose mode or allow-default.
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 requires the reverse path to be via the same interface; asymmetric routing causes drops.
Why this is correct
Strict mode fails if the return route uses a different interface, which is common in DMVPN.
Clue confirmation
The clue words "best", "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The hub's IP address is not in the routing table, so uRPF drops the packet.
Why it's wrong here
The hub is reachable, but via a different path.
✗
uRPF must be configured with the `allow-default` option to accept packets with default route.
Why it's wrong here
Allow-default helps if the default route is used, but the issue is asymmetric routing.
✗
uRPF is not supported on tunnel interfaces; it must be applied on the physical interface.
Why it's wrong here
uRPF is supported on tunnel interfaces.
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.
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — 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 requires the reverse path to be via the same interface; asymmetric routing causes drops. — uRPF strict mode checks that the source IP of incoming packets has a route back to the source via the same interface. In DMVPN, the hub's source IP may be reachable via a different interface (e.g., physical interface) than the tunnel interface where the packet arrives. This asymmetric routing causes uRPF strict mode to drop the packets. The solution is to use loose mode or allow-default.
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: "best", "most likely". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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 →
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.
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.
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.
Sign in to join the discussion.