The answer is that R2’s tunnel interface has an inbound ACL denying ICMP, which blocks R1’s ping requests. This occurs because the ACL is applied inbound on R2’s tunnel interface, so when R1 sends an ICMP echo request to R2’s tunnel IP, the packet is evaluated and denied upon arrival. R2 can still ping R1 because outbound traffic from R2 is not inspected by that inbound ACL. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of DMVPN tunnel ACL directionality and how it interacts with EIGRP spoke-to-hub reachability. A common trap is assuming ACLs on tunnel interfaces affect both directions equally, but the key is that inbound ACLs filter traffic entering the interface, not leaving it. Remember the memory tip: “Inbound blocks what comes in, not what goes out”—so if a spoke cannot be pinged, check the inbound ACL on its tunnel interface first.
300-410 DMVPN Practice Question
This 300-410 practice question tests your understanding of dmvpn. 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.
In a DMVPN Phase 2 network with EIGRP, R1 (hub) and R2 (spoke) are configured. R2's tunnel interface has an ACL applied inbound that denies ICMP. R2 can ping R1's tunnel IP, but R1 cannot ping R2's tunnel IP. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R2's tunnel interface has an inbound ACL that denies ICMP, blocking R1's ping requests.
The ACL on R2's tunnel interface inbound denies ICMP. When R1 sends an ICMP echo request to R2, it is denied by the ACL. However, R2 can ping R1 because the ACL does not affect outbound traffic. The issue is that the ACL is applied inbound, blocking incoming ICMP packets.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
R2's tunnel interface has an inbound ACL that denies ICMP, blocking R1's ping requests.
Why this is correct
Correct. Inbound ACLs filter traffic entering the interface. ICMP echo requests from R1 are denied, so R1 cannot ping R2.
Related concept
Standard ACLs match source addresses.
✗
R1's tunnel interface has an outbound ACL that denies ICMP.
Why it's wrong here
Incorrect. An outbound ACL on R1 would block traffic leaving R1, but R2 can ping R1, so outbound traffic is not blocked.
✗
NHRP is not resolving R2's tunnel IP to its physical IP on R1.
Why it's wrong here
Incorrect. If R2 can ping R1, NHRP resolution is working.
✗
EIGRP is not advertising R2's tunnel IP to R1.
Why it's wrong here
Incorrect. R2 can ping R1, so routing is working.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
DMVPN — This question tests DMVPN — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: R2's tunnel interface has an inbound ACL that denies ICMP, blocking R1's ping requests. — The ACL on R2's tunnel interface inbound denies ICMP. When R1 sends an ICMP echo request to R2, it is denied by the ACL. However, R2 can ping R1 because the ACL does not affect outbound traffic. The issue is that the ACL is applied inbound, blocking incoming ICMP packets.
What should I do if I get this 300-410 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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