Question 826 of 2,152

Device Management →mediumMultiple ChoiceObjective-mapped

ICMP Blocked by Interface ACL

This 300-410 practice question tests your understanding of device management. 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 network engineer is troubleshooting a router that is not responding to ICMP echo requests from a management station at 10.10.10.1. The router has an ACL applied to the VTY lines that permits only 10.10.10.0/24. The engineer can telnet to the router from the management station. What is the most likely cause?

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.

A
The VTY ACL also applies to ICMP traffic.
Why wrong: VTY ACLs only control Telnet/SSH access, not ICMP.
B
An inbound ACL on the interface denies ICMP from 10.10.10.1.
Since Telnet works, the VTY ACL is not the issue; an interface ACL blocking ICMP is the likely cause.
C
The router has 'no ip icmp echo' configured globally.
Why wrong: There is no such global command; ICMP echo responses are controlled by interface ACLs or the 'ip icmp rate-limit' feature.
D
The management station is not in the routing table of the router.
Why wrong: If the management station were not reachable, Telnet would also fail.

Full breakdown with real-world context →

Quick Answer

The answer is an inbound ACL on the interface that denies ICMP from 10.10.10.1. This is correct because Telnet and ICMP are controlled by different ACL application points: Telnet traffic is filtered by the VTY access-class applied to the router’s management lines, while ICMP echo requests are processed by the interface ACL applied inbound on the router’s physical or virtual interface. Since the management station can telnet successfully, the VTY ACL is correctly permitting 10.10.10.0/24, but the router’s failure to respond to pings indicates that an interface ACL is blocking ICMP before it reaches the control plane. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding of ACL placement and the distinction between management-plane and data-plane filtering—a common trap is assuming a VTY ACL controls all traffic to the router. Remember the memory tip: “Telnet talks to VTY, but ICMP hits the interface first.”

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

✓

An inbound ACL on the interface denies ICMP from 10.10.10.1.

The management station can telnet to the router, which proves that the VTY ACL permits 10.10.10.0/24 and that reachability exists. Since ICMP is processed by the interface input path before any VTY ACL, the failure of ICMP echo requests points to an inbound ACL on the interface that explicitly denies ICMP from 10.10.10.1. This is the most likely cause because telnet (TCP/23) succeeds while ICMP fails, isolating the issue to interface-level filtering.

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 VTY ACL also applies to ICMP traffic.
Why it's wrong here
VTY ACLs only control Telnet/SSH access, not ICMP.
✓
An inbound ACL on the interface denies ICMP from 10.10.10.1.
Why this is correct
Since Telnet works, the VTY ACL is not the issue; an interface ACL blocking ICMP is the likely cause.
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 router has 'no ip icmp echo' configured globally.
Why it's wrong here
There is no such global command; ICMP echo responses are controlled by interface ACLs or the 'ip icmp rate-limit' feature.
✗
The management station is not in the routing table of the router.
Why it's wrong here
If the management station were not reachable, Telnet would also fail.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that VTY ACLs control all management traffic, including ICMP, when in fact VTY ACLs only filter Telnet/SSH sessions, while ICMP is filtered by interface ACLs.

Trap categories for this question

Command / output trap
There is no such global command; ICMP echo responses are controlled by interface ACLs or the 'ip icmp rate-limit' feature.

Detailed technical explanation

How to think about this question

ICMP echo requests are processed by the router's interface input ACL before any routing decision or VTY access. If an inbound ACL on the interface denies ICMP from a specific source, the packet is dropped immediately, while TCP traffic (like Telnet) may be permitted by a different ACE. This is a common troubleshooting scenario where 'ping fails but telnet works' isolates the issue to interface ACLs rather than VTY ACLs or routing.

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 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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: An inbound ACL on the interface denies ICMP from 10.10.10.1. — The management station can telnet to the router, which proves that the VTY ACL permits 10.10.10.0/24 and that reachability exists. Since ICMP is processed by the interface input path before any VTY ACL, the failure of ICMP echo requests points to an inbound ACL on the interface that explicitly denies ICMP from 10.10.10.1. This is the most likely cause because telnet (TCP/23) succeeds while ICMP fails, isolating the issue to interface-level filtering.

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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