The answer is that the ACL is filtering the outer IP header of the mGRE tunnel, blocking spoke-to-spoke traffic before it can be decapsulated. In a DMVPN Phase 2 network, when a spoke sends traffic directly to another spoke, the router encapsulates the original packet with a new outer IP header whose destination is the target spoke’s public address. An inbound ACL applied to the tunnel interface inspects packets after decapsulation—meaning it sees the inner, original header—but if the ACL is mistakenly applied to the physical interface or if the tunnel interface ACL is misconfigured to match the outer header, it drops the encapsulated packets before NHRP can process them. This is a classic trap on the Cisco CCNP ENARSI 300-410 exam: candidates often assume NHRP success guarantees data flow, but the ACL blocks the outer encapsulation, not the inner routing. Remember the key distinction: NHRP resolves the destination, but the ACL filters the transport wrapper. Memory tip: “Outer blocks, inner talks—check which header your ACL walks.”
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
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?
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 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.
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.
✓
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In DMVPN Phase 2, spoke-to-spoke tunnels use mGRE encapsulation where the outer IP header carries the public IP of the destination spoke. An inbound ACL on the tunnel interface inspects the inner packet after decapsulation, but if the ACL is applied on the physical interface (e.g., GigabitEthernet) instead, it filters the outer header. A common misconfiguration is applying the ACL to the tunnel interface itself but with incorrect direction or entries that match the outer source IP, which is the hub's IP (since traffic is routed through the hub initially), causing the spoke to drop encapsulated packets from other spokes. The show ip access-lists command can verify if the ACL is matching the outer or inner headers.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
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.
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?
Read the scenario before looking for a memorised answer.
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