Question 530 of 2,152

BGP Troubleshooting →hardMultiple ChoiceObjective-mapped

CoPP Impact on BGP Sessions — Deny Class-Default | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of bgp troubleshooting. 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) with a policy that denies all traffic in class-default. After applying the policy, BGP sessions to the router fail. 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 has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets.
Explicit deny in class-default changes the default behavior from permit to drop.
B
The CoPP policy was applied to the wrong interface, so BGP packets are dropped by the interface ACL.
Why wrong: CoPP is applied to the control plane, not interfaces.
C
The BGP packets are matched by another class with a 'drop' action, but the class-default is irrelevant.
Why wrong: If another class drops BGP, that would be the cause, but the question specifies class-default deny.
D
The CoPP policy uses 'rate-limit' in bps instead of pps, causing BGP packets to be dropped due to rate limiting.
Why wrong: Rate limiting would cause drops only if exceeded, not a complete failure.

Full breakdown with real-world context →

Quick Answer

The answer is that the explicit deny action in class-default overrides the implicit permit, causing CoPP to drop all unmatched traffic, including BGP packets. Control Plane Policing (CoPP) operates with an implicit permit at the end of the class-default by default, meaning any traffic not matched by a higher-priority class is allowed through. However, when an engineer explicitly configures a deny action within class-default, that explicit drop takes precedence over the implicit permit, effectively blackholing all control-plane traffic—including BGP session packets—that isn’t caught by a more specific class. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of CoPP policy evaluation order and the critical distinction between implicit and explicit actions in class-default. A common trap is assuming the implicit permit always applies, but remember: an explicit deny in class-default is absolute. Memory tip: “Explicit beats implicit—if you drop the default, you drop the BGP.”

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 has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets.

CoPP class-default has an implicit permit at the end, but if an explicit deny is configured in class-default, it will drop all traffic not matched by other classes, including BGP control packets. The explicit deny overrides the implicit permit.

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 has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets.
Why this is correct
Explicit deny in class-default changes the default behavior from permit to drop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The CoPP policy was applied to the wrong interface, so BGP packets are dropped by the interface ACL.
Why it's wrong here
CoPP is applied to the control plane, not interfaces.
✗
The BGP packets are matched by another class with a 'drop' action, but the class-default is irrelevant.
Why it's wrong here
If another class drops BGP, that would be the cause, but the question specifies class-default deny.
✗
The CoPP policy uses 'rate-limit' in bps instead of pps, causing BGP packets to be dropped due to rate limiting.
Why it's wrong here
Rate limiting would cause drops only if exceeded, not a complete failure.

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.

Visual reference

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?

BGP Troubleshooting — This question tests BGP Troubleshooting — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The class-default has an explicit 'drop' action, which overrides the implicit permit and drops all unmatched traffic, including BGP packets. — CoPP class-default has an implicit permit at the end, but if an explicit deny is configured in class-default, it will drop all traffic not matched by other classes, including BGP control packets. The explicit deny overrides the implicit permit.

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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