Question 438 of 2,152

MPLS Operations →hardMultiple ChoiceObjective-mapped

300-410 MPLS Operations Practice Question

This 300-410 practice question tests your understanding of mpls operations. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 Control Plane Policing (CoPP) on a router to protect the control plane. After applying the policy, the router becomes unreachable via SSH and SNMP. The engineer checks the policy and confirms that the class-map for SSH and SNMP traffic is set to 'permit'. 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.

A
The class-default is set to 'drop', causing all unmatched traffic to be dropped, including SSH and SNMP if they are not correctly classified.
In CoPP, the class-default is the default class for all traffic not matched by other classes. If it is set to drop, any traffic that does not match the explicit classes will be dropped. If SSH or SNMP traffic is not correctly matched by the class-map (e.g., due to a typo in the access-list), it will fall into class-default and be dropped.
B
The 'rate-limit' is configured in bps instead of pps, causing excessive policing.
Why wrong: While unit mismatch can cause issues, it would not cause complete unreachability unless the rate is set to 0. The question states the policy is applied, but the router becomes unreachable, suggesting dropping rather than policing.
C
The 'service-policy' is applied to the control-plane input direction, but SSH and SNMP are output traffic.
Why wrong: SSH and SNMP traffic to the router is input traffic to the control plane, so applying CoPP input is correct. Output CoPP is for traffic originated by the router.
D
The class-map for SSH and SNMP uses a 'match-all' condition, but the access-list has multiple entries that are ORed.
Why wrong: Using 'match-all' with an access-list is fine; the access-list entries are ORed by default. This would not cause the traffic to be dropped.

Full breakdown with real-world context →

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 class-default is set to 'drop', causing all unmatched traffic to be dropped, including SSH and SNMP if they are not correctly classified.

A common edge case with CoPP is that the default class class-default is often set to 'drop' or 'police' with a very low rate, which can drop all traffic not explicitly matched by other classes. Even if SSH and SNMP are permitted, if the class-default is set to drop, any traffic that does not match the explicit classes (e.g., due to a misclassification) will be dropped. Additionally, the order of class-maps matters; if a broader class matches before the specific one, the traffic may be policed incorrectly.

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.

✓
The class-default is set to 'drop', causing all unmatched traffic to be dropped, including SSH and SNMP if they are not correctly classified.
Why this is correct
In CoPP, the class-default is the default class for all traffic not matched by other classes. If it is set to drop, any traffic that does not match the explicit classes will be dropped. If SSH or SNMP traffic is not correctly matched by the class-map (e.g., due to a typo in the access-list), it will fall into class-default and be dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The 'rate-limit' is configured in bps instead of pps, causing excessive policing.
Why it's wrong here
While unit mismatch can cause issues, it would not cause complete unreachability unless the rate is set to 0. The question states the policy is applied, but the router becomes unreachable, suggesting dropping rather than policing.
✗
The 'service-policy' is applied to the control-plane input direction, but SSH and SNMP are output traffic.
Why it's wrong here
SSH and SNMP traffic to the router is input traffic to the control plane, so applying CoPP input is correct. Output CoPP is for traffic originated by the router.
✗
The class-map for SSH and SNMP uses a 'match-all' condition, but the access-list has multiple entries that are ORed.
Why it's wrong here
Using 'match-all' with an access-list is fine; the access-list entries are ORed by default. This would not cause the traffic to be dropped.

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.

Trap categories for this question

Command / output trap
SSH and SNMP traffic to the router is input traffic to the control plane, so applying CoPP input is correct. Output CoPP is for traffic originated by the router.

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.

What to study next

Got this wrong? Here's your next step.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

MPLS Operations — This question tests MPLS Operations — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The class-default is set to 'drop', causing all unmatched traffic to be dropped, including SSH and SNMP if they are not correctly classified. — A common edge case with CoPP is that the default class class-default is often set to 'drop' or 'police' with a very low rate, which can drop all traffic not explicitly matched by other classes. Even if SSH and SNMP are permitted, if the class-default is set to drop, any traffic that does not match the explicit classes (e.g., due to a misclassification) will be dropped. Additionally, the order of class-maps matters; if a broader class matches before the specific one, the traffic may be policed incorrectly.

What should I do if I get this 300-410 question wrong?

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?

Standard ACLs match source addresses.

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