Question 766 of 2,152

SNMP Troubleshooting →hardMultiple ChoiceObjective-mapped

CoPP Blocking SNMP — Troubleshooting

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.

An engineer configures SNMPv2c with a community string 'public' and an ACL that permits the NMS. The NMS can poll the router. The engineer then applies a CoPP policy that drops SNMP packets (UDP port 161) from all sources except the NMS. The NMS now fails to poll. Which 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.

A
The CoPP policy's ACL does not include the NMS IP address, so SNMP packets from the NMS are dropped by the class-default.
CoPP policies typically have a class that matches traffic to be permitted; if the NMS is not matched, its packets fall into class-default, which may be set to drop.
B
CoPP only affects routing protocol traffic, not SNMP.
Why wrong: CoPP can filter any control plane traffic, including SNMP.
C
The SNMP community string ACL is overridden by the CoPP policy.
Why wrong: The community string ACL and CoPP are independent; both must permit the traffic.
D
The router requires a reload for the CoPP policy to take effect.
Why wrong: CoPP policies are applied immediately.

Full breakdown with real-world context →

Quick Answer

The answer is that the CoPP policy’s ACL does not include the NMS IP address, so SNMP packets from the NMS are dropped by the class-default. CoPP applies to control plane traffic, meaning any SNMP polling reaching the router’s control plane must match a permit entry in the CoPP class map; if the ACL is misconfigured or missing the NMS source, the traffic falls into class-default, which typically drops all unmatched packets. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that CoPP operates independently from SNMP’s own ACL—a common trap is assuming the SNMP community-string ACL alone protects the NMS, but CoPP can override it. Remember the memory tip: “CoPP is the bouncer; if the bouncer’s list is wrong, even VIPs get turned away.” Always verify that the CoPP ACL explicitly permits the NMS source IP, and check the class-default action to avoid silent drops.

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 CoPP policy's ACL does not include the NMS IP address, so SNMP packets from the NMS are dropped by the class-default.

Option A is correct because CoPP (Control Plane Policing) applies to all traffic destined to the router's control plane, including SNMP. If the CoPP policy's ACL does not explicitly permit the NMS IP address for UDP port 161, those packets fall into the class-default, which is configured to drop them. Even though the SNMP community string ACL permits the NMS, CoPP is evaluated before the packet reaches the SNMP process, so the drop occurs first.

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 CoPP policy's ACL does not include the NMS IP address, so SNMP packets from the NMS are dropped by the class-default.
Why this is correct
CoPP policies typically have a class that matches traffic to be permitted; if the NMS is not matched, its packets fall into class-default, which may be set to drop.
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.
✗
CoPP only affects routing protocol traffic, not SNMP.
Why it's wrong here
CoPP can filter any control plane traffic, including SNMP.
✗
The SNMP community string ACL is overridden by the CoPP policy.
Why it's wrong here
The community string ACL and CoPP are independent; both must permit the traffic.
✗
The router requires a reload for the CoPP policy to take effect.
Why it's wrong here
CoPP policies are applied immediately.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the order of operations: CoPP is applied before the SNMP process, so a CoPP drop will prevent SNMP from ever seeing the packet, regardless of SNMP ACLs or community strings.

Detailed technical explanation

How to think about this question

CoPP uses MQC (Modular QoS CLI) to classify traffic with ACLs and apply policing actions (drop, transmit, or rate-limit) to control plane packets. The control plane is the CPU's path for management and routing protocols; SNMP packets (UDP 161) are processed here. A common misconfiguration is forgetting to include the NMS IP in the CoPP permit ACL, causing all SNMP traffic to hit the default drop class, even if SNMP itself is correctly configured.

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

Quick reference

IPv4 Address Class Summary

Class	First Octet Range	Default Mask	Networks	Hosts per Network
A	1–126	/8 (255.0.0.0)	126	16,777,214
B	128–191	/16 (255.255.0.0)	16,384	65,534
C	192–223	/24 (255.255.255.0)	2,097,152	254
D	224–239	N/A	Multicast groups	—
E	240–255	N/A	Reserved / experimental	—

127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.

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?

SNMP Troubleshooting — This question tests SNMP Troubleshooting — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The CoPP policy's ACL does not include the NMS IP address, so SNMP packets from the NMS are dropped by the class-default. — Option A is correct because CoPP (Control Plane Policing) applies to all traffic destined to the router's control plane, including SNMP. If the CoPP policy's ACL does not explicitly permit the NMS IP address for UDP port 161, those packets fall into the class-default, which is configured to drop them. Even though the SNMP community string ACL permits the NMS, CoPP is evaluated before the packet reaches the SNMP process, so the drop occurs first.

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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