- A
The spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address.
Correct. uRPF loose mode requires that the source address be present in the routing table (any interface). If the address is only in NHRP cache, not in the routing table, packets are dropped.
- B
The hub has uRPF strict mode configured, which breaks the DMVPN tunnel because of asymmetric routing.
Why wrong: Incorrect. The question specifies uRPF loose mode on the spoke interface, not strict mode on the hub.
- C
The spoke's NHRP registration packets are filtered by the uRPF check because they use multicast destination.
Why wrong: Incorrect. NHRP uses unicast or multicast; uRPF checks source address, not destination.
- D
The tunnel interface has an IPv6 ACL that denies traffic from the spoke's tunnel address, overriding uRPF.
Why wrong: Incorrect. There is no mention of an ACL; the issue is uRPF behavior.
Quick Answer
The answer is that the spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address. In uRPF loose mode, the router verifies that the source address of an incoming packet has at least one routing table entry, regardless of the interface; however, if the spoke’s tunnel IPv6 address is learned exclusively through NHRP and not injected into the routing table via a dynamic protocol like OSPFv3 or EIGRP, that source address will appear unreachable, and the packet is discarded. This scenario is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding of how uRPF interacts with DMVPN’s reliance on NHRP for address resolution rather than traditional routing. A common memory tip is “NHRP is not a routing protocol”—just because the hub knows the spoke’s tunnel address via NHRP does not mean the spoke’s own router has a route to that address, so uRPF loose mode sees it as a source with no valid path and drops the traffic.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 IPv6 uRPF loose mode on an interface that connects to a DMVPN spoke. The spoke router uses NHRP to register with the hub and establishes a tunnel. Traffic from the spoke to destinations behind the hub is dropped. 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
The spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address.
In DMVPN Phase 2 or 3, spoke-to-spoke traffic may use direct tunnels. With uRPF loose mode, the router checks that the source address of incoming packets has a routing table entry (any interface). However, if the spoke's tunnel interface uses an IPv6 address that is not advertised via routing protocols (e.g., only NHRP), the source address may not be in the routing table, causing uRPF loose mode to drop the packet. Additionally, asymmetric routing is common in DMVPN, and uRPF loose mode may still drop if the source address is not reachable at all.
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 spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address.
Why this is correct
Correct. uRPF loose mode requires that the source address be present in the routing table (any interface). If the address is only in NHRP cache, not in the routing table, packets are dropped.
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 hub has uRPF strict mode configured, which breaks the DMVPN tunnel because of asymmetric routing.
Why it's wrong here
Incorrect. The question specifies uRPF loose mode on the spoke interface, not strict mode on the hub.
- ✗
The spoke's NHRP registration packets are filtered by the uRPF check because they use multicast destination.
Why it's wrong here
Incorrect. NHRP uses unicast or multicast; uRPF checks source address, not destination.
- ✗
The tunnel interface has an IPv6 ACL that denies traffic from the spoke's tunnel address, overriding uRPF.
Why it's wrong here
Incorrect. There is no mention of an ACL; the issue is uRPF behavior.
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?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address. — In DMVPN Phase 2 or 3, spoke-to-spoke traffic may use direct tunnels. With uRPF loose mode, the router checks that the source address of incoming packets has a routing table entry (any interface). However, if the spoke's tunnel interface uses an IPv6 address that is not advertised via routing protocols (e.g., only NHRP), the source address may not be in the routing table, causing uRPF loose mode to drop the packet. Additionally, asymmetric routing is common in DMVPN, and uRPF loose mode may still drop if the source address is not reachable at all.
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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