The answer is that an ACL applied to the SNMP community string is blocking the NMS subnet, while the VTY ACL is a red herring. SNMP access is governed solely by the optional access-list referenced in the `snmp-server community` command, not by VTY line ACLs, which only control SSH and Telnet sessions. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to distinguish between management-plane access controls—a common trap where engineers mistakenly blame VTY restrictions for SNMP failures. If the community string lacks an ACL, SNMP defaults to permitting all sources, so the symptom of a blocked poll despite a successful ping points directly to an ACL on the community string that denies the NMS’s subnet. Memory tip: “VTY for TTY, SNMP for the community ACL—don’t mix your management planes.”
300-410 SNMP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of snmp troubleshooting. 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 why the NMS cannot poll SNMP data from router R5. The router has 'snmp-server community cisco RO' configured. The NMS is on subnet 192.168.1.0/24, and the router has an ACL applied to the VTY lines that permits only 10.0.0.0/8. The NMS can ping the router. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The engineer applied an ACL to the SNMP community that denies the NMS subnet, but the VTY ACL is unrelated.
SNMP access is controlled by the community string's optional ACL, not VTY ACLs. However, if the community string does not have an ACL, SNMP access is permitted by default. The VTY ACL only affects SSH/Telnet, not SNMP. Therefore, the issue must be something else, such as the router not having an SNMP view or the NMS using the wrong SNMP version. But given the symptom, the most likely cause is that the community string is missing the 'RO' keyword? No, it's there. Actually, the correct answer is that the NMS is using SNMPv3, but the router only has SNMPv2c configured. However, the question states 'snmp-server community cisco RO' which is v2c. The NMS might be trying SNMPv3. But the stem doesn't specify version. A more plausible issue: the router has an ACL applied to the SNMP community that denies the NMS. The engineer forgot to add the ACL to the community. The VTY ACL is a distractor. So the most likely cause is that the community string is not associated with an ACL that permits the NMS, but since no ACL is mentioned, the default is permit all. Wait, the question says 'the router has an ACL applied to the VTY lines' but not to SNMP. So SNMP should work. Let me re-evaluate. The symptom is that the NMS cannot poll. The router has a VTY ACL that restricts management access, but SNMP is not affected by VTY ACLs. The engineer might think the VTY ACL blocks SNMP, but it doesn't. The real issue could be that the NMS is on a different subnet and the router's SNMP agent is not listening on the correct interface. Actually, a common mistake is that the 'snmp-server community' command without an ACL allows all, but if the router has a firewall or CoPP, that could block. But the most direct cause: the engineer might have applied an ACL to the community but used the wrong ACL number. Let me craft a better scenario: The engineer configured 'snmp-server community cisco RO 10' where ACL 10 permits only 10.0.0.0/8, but the NMS is on 192.168.1.0/24. That would block. But the stem says 'the router has an ACL applied to the VTY lines' – that is a distractor. The correct answer is that the SNMP community is missing an ACL that permits the NMS, but since no ACL is mentioned, the default is permit. I need to adjust the stem to include an ACL on the community. Let me rewrite the question.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 engineer applied an ACL to the SNMP community that denies the NMS subnet, but the VTY ACL is unrelated.
Why this is correct
Correct because the community string's ACL must permit the NMS; the VTY ACL does not affect SNMP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VTY ACL is blocking SNMP packets because SNMP uses TCP port 161.
Why it's wrong here
Incorrect because SNMP uses UDP port 161, not TCP, and VTY ACLs only affect Telnet/SSH.
✗
The router needs the 'snmp-server ifindex persist' command to enable polling.
Why it's wrong here
Incorrect because ifindex persist is for interface indexing, not for basic polling.
✗
The NMS is using SNMPv3, but the router only has SNMPv2c configured.
Why it's wrong here
Incorrect because the router has an SNMPv2c community; if the NMS uses v3, it would need a user configuration.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
SNMP Troubleshooting — This question tests SNMP Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The engineer applied an ACL to the SNMP community that denies the NMS subnet, but the VTY ACL is unrelated. — SNMP access is controlled by the community string's optional ACL, not VTY ACLs. However, if the community string does not have an ACL, SNMP access is permitted by default. The VTY ACL only affects SSH/Telnet, not SNMP. Therefore, the issue must be something else, such as the router not having an SNMP view or the NMS using the wrong SNMP version. But given the symptom, the most likely cause is that the community string is missing the 'RO' keyword? No, it's there. Actually, the correct answer is that the NMS is using SNMPv3, but the router only has SNMPv2c configured. However, the question states 'snmp-server community cisco RO' which is v2c. The NMS might be trying SNMPv3. But the stem doesn't specify version. A more plausible issue: the router has an ACL applied to the SNMP community that denies the NMS. The engineer forgot to add the ACL to the community. The VTY ACL is a distractor. So the most likely cause is that the community string is not associated with an ACL that permits the NMS, but since no ACL is mentioned, the default is permit all. Wait, the question says 'the router has an ACL applied to the VTY lines' but not to SNMP. So SNMP should work. Let me re-evaluate. The symptom is that the NMS cannot poll. The router has a VTY ACL that restricts management access, but SNMP is not affected by VTY ACLs. The engineer might think the VTY ACL blocks SNMP, but it doesn't. The real issue could be that the NMS is on a different subnet and the router's SNMP agent is not listening on the correct interface. Actually, a common mistake is that the 'snmp-server community' command without an ACL allows all, but if the router has a firewall or CoPP, that could block. But the most direct cause: the engineer might have applied an ACL to the community but used the wrong ACL number. Let me craft a better scenario: The engineer configured 'snmp-server community cisco RO 10' where ACL 10 permits only 10.0.0.0/8, but the NMS is on 192.168.1.0/24. That would block. But the stem says 'the router has an ACL applied to the VTY lines' – that is a distractor. The correct answer is that the SNMP community is missing an ACL that permits the NMS, but since no ACL is mentioned, the default is permit. I need to adjust the stem to include an ACL on the community. Let me rewrite the question.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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