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300-410 Practice Question: A DMVPN network uses IPv6 with EIGRP as the…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.