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300-410 Practice Question: On a DMVPN Phase 2 hub-and-spoke network, an…
On a DMVPN Phase 2 hub-and-spoke network, an engineer applies an inbound IPv4 ACL on the tunnel interface of a spoke router to permit only traffic from specific spoke IPs. After the ACL is applied, the spoke cannot establish a direct spoke-to-spoke tunnel with another spoke, even though NHRP resolution succeeds. What is the most likely explanation?
⚠ Common exam trap
Cisco often tests the distinction between filtering the outer IP header (physical interface) versus the inner IP header (tunnel interface) in DMVPN, leading candidates to incorrectly assume that an ACL on the tunnel interface always inspects the inner packet, when in fact the ACL placement and direction determine which header is evaluated.
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 ACL is filtering the outer IP header of the mGRE tunnel, blocking spoke-to-spoke traffic.
In a DMVPN Phase 2 network, spoke-to-spoke traffic is encapsulated in a new IP header with the destination IP set to the target spoke's public address. When an inbound ACL is applied to the spoke's tunnel interface, it filters traffic after decapsulation, meaning it inspects the inner (original) IP header. However, the ACL is applied on the physical interface or the tunnel interface in a way that inadvertently filters the outer IP header of the mGRE tunnel, blocking the encapsulated spoke-to-spoke packets before they can be processed. Since NHRP resolution succeeds, the spoke knows the peer's address, but the actual data traffic is dropped because the ACL denies the outer encapsulation.
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 ACL is filtering the outer IP header of the mGRE tunnel, blocking spoke-to-spoke traffic.
Why this is correct
The ACL on the tunnel interface filters the encapsulated packets; if it only permits the hub's IP, traffic from other spokes is denied.
- ✗
The ACL is filtering NHRP packets, preventing spoke-to-spoke resolution.
Why it's wrong here
NHRP resolution succeeds, so NHRP packets are not blocked.
- ✗
The spoke has a misconfigured NHRP authentication key.
Why it's wrong here
Authentication mismatch would prevent NHRP registration, but resolution succeeds.
- ✗
The ACL is applied outbound, filtering outgoing traffic.
Why it's wrong here
Outbound ACL would affect traffic leaving the spoke, not incoming spoke-to-spoke traffic.
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Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
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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.