- A
R1's inbound ACL on the tunnel interface permits only EIGRP, dropping all other traffic including spoke-to-spoke data packets.
Spoke-to-spoke traffic is forwarded through the hub if the routing table points to the hub. The ACL on the hub's tunnel interface filters this traffic.
- B
NAT traversal is broken for spoke-to-spoke traffic due to IPsec encryption issues.
Why wrong: The ping to the hub works, indicating NAT traversal is functional.
- C
EIGRP is not advertising spoke loopbacks to other spokes, causing no route.
Why wrong: If EIGRP were not advertising, the ping would fail with 'no route', but the issue is filtering.
- D
The mGRE tunnel on R2 does not have a destination for R3, preventing direct communication.
Why wrong: DMVPN dynamically creates spoke-to-spoke tunnels; the mGRE configuration is correct.
Quick Answer
The answer is that the hub router’s inbound IPv6 ACL on the tunnel interface is the root cause, because it permits only EIGRP and denies all other IPv6 traffic, including spoke-to-spoke data packets. Even though R2 and R3 establish a direct DMVPN tunnel between themselves, the traffic still traverses the hub’s tunnel interface when the routing table points to the hub as the next hop—a common scenario when spoke-to-spoke routes are not dynamically optimized via NHRP shortcuts. This question tests your understanding of how DMVPN tunnel ACL filtering interacts with IPv6 traffic flows on the Cisco CCNP ENARSI 300-410 exam, often catching candidates who assume a direct tunnel bypasses hub policies. The trap is forgetting that the hub’s inbound ACL applies to all traffic entering its tunnel interface, regardless of the ultimate source or destination. Memory tip: on DMVPN, the hub’s ACL is a gatekeeper—even spoke-to-spoke packets must knock before passing through.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 DMVPN network uses IPv6 with EIGRP as the routing protocol. Spoke routers R2 and R3 are behind NAT and use mGRE tunnels. The hub R1 has an IPv6 ACL applied inbound on the tunnel interface that permits only EIGRP and denies all other IPv6 traffic. Spoke-to-spoke traffic fails even though direct tunnels are established. R2 shows 'ping 2001:db8:3::1 source loopback0' fails, but 'ping 2001:db8:1::1' (hub) succeeds. What is the root cause?
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
R1's inbound ACL on the tunnel interface permits only EIGRP, dropping all other traffic including spoke-to-spoke data packets.
The ACL on R1's tunnel interface blocks spoke-to-spoke traffic because it is not sourced from the hub. Even though the spokes have a direct tunnel, the traffic still traverses the hub's tunnel interface if the routing is not optimized. The ACL permits only EIGRP, so data traffic is dropped.
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.
- ✓
R1's inbound ACL on the tunnel interface permits only EIGRP, dropping all other traffic including spoke-to-spoke data packets.
- ✗
NAT traversal is broken for spoke-to-spoke traffic due to IPsec encryption issues.
Why it's wrong here
The ping to the hub works, indicating NAT traversal is functional.
- ✗
EIGRP is not advertising spoke loopbacks to other spokes, causing no route.
Why it's wrong here
If EIGRP were not advertising, the ping would fail with 'no route', but the issue is filtering.
- ✗
The mGRE tunnel on R2 does not have a destination for R3, preventing direct communication.
Why it's wrong here
DMVPN dynamically creates spoke-to-spoke tunnels; the mGRE configuration is correct.
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: R1's inbound ACL on the tunnel interface permits only EIGRP, dropping all other traffic including spoke-to-spoke data packets. — The ACL on R1's tunnel interface blocks spoke-to-spoke traffic because it is not sourced from the hub. Even though the spokes have a direct tunnel, the traffic still traverses the hub's tunnel interface if the routing is not optimized. The ACL permits only EIGRP, so data traffic is dropped.
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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Last reviewed: Jun 18, 2026
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.
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