- 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.
DMVPN Hub ACL Filtering Spoke-to-Spoke IPv6 Traffic
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?
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.
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 hub router R1 has an inbound IPv6 ACL on its tunnel interface that permits only EIGRP traffic and denies all other IPv6 traffic. When spoke R2 attempts to send data (e.g., ping) directly to spoke R3, the packets are routed through the hub because the spokes do not have a direct route to each other's loopback addresses. Even though a direct mGRE tunnel exists between spokes, the data packets must first reach the hub, which drops them due to the ACL, causing spoke-to-spoke communication failure.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Why this is correct
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.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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: answer the scenario, not the keyword
Cisco often tests the misconception that a direct tunnel between spokes automatically allows direct traffic, but in reality, the hub's ACL or routing policy can still block spoke-to-spoke data if the packets are forwarded through the hub.
Detailed technical explanation
How to think about this question
In a DMVPN phase 2 or 3 network, spoke-to-spoke traffic is initially forwarded through the hub until the spokes learn each other's NHRP mappings and install direct routes. However, even with direct tunnels, the hub's inbound ACL can intercept traffic if the routing table points to the hub as the next hop (e.g., due to summarization or split horizon). The ACL applied inbound on the hub's tunnel interface filters all IPv6 traffic except EIGRP, meaning any data packet (like ICMPv6) arriving from a spoke is dropped, preventing spoke-to-spoke communication that relies on hub forwarding.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
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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 — Read the scenario before looking for a memorised answer..
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 hub router R1 has an inbound IPv6 ACL on its tunnel interface that permits only EIGRP traffic and denies all other IPv6 traffic. When spoke R2 attempts to send data (e.g., ping) directly to spoke R3, the packets are routed through the hub because the spokes do not have a direct route to each other's loopback addresses. Even though a direct mGRE tunnel exists between spokes, the data packets must first reach the hub, which drops them due to the ACL, causing spoke-to-spoke communication failure.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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