Question 471 of 2,015

ACLs and CoPP →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the ACL denies all other BGP traffic, so CoPP does not match it, and it falls through to the default class, which permits it. This occurs because CoPP ACL classification works by matching only permitted entries in the access list; a deny entry does not drop traffic but instead causes the packet to skip that class-map and be evaluated by the default class. In the ENCOR 350-401 exam, this scenario tests your understanding that CoPP class-maps use permit statements to select traffic for policing, while any traffic not explicitly permitted is handled by the default class, which typically allows all traffic unless explicitly configured otherwise. A common trap is assuming that a deny in the ACL will drop the traffic, but CoPP ACLs are for classification only—actual policing is done by the policy-map’s action. Remember the memory tip: “Deny in CoPP is just a skip, not a drop.”

350-401 ACLs and CoPP Practice Question

This 350-401 practice question tests your understanding of acls and copp. 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 configuring CoPP on a Cisco ASR 1000 router to protect the control plane from excessive traffic. The engineer wants to allow BGP traffic from a specific peer (10.0.0.1) while rate-limiting all other BGP traffic. The engineer creates an ACL that permits TCP port 179 from host 10.0.0.1 and denies all other BGP traffic. The CoPP class-map matches this ACL. However, after applying the policy, BGP sessions from other peers are still being established. What is the most likely reason?

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.

Question 1hardmultiple choice

Open the full BGP breakdown →

A
The ACL denies all other BGP traffic, so CoPP does not match it, and it falls through to the default class, which permits it.
Correct because CoPP only applies to traffic matched by the class-map; if the ACL denies traffic, it is not matched, and the default class (often permit) allows it.
B
The ACL is applied in the wrong order; the deny statement should be before the permit statement.
Why wrong: Incorrect because ACL order is evaluated sequentially; a permit before deny would permit the specific peer, but the deny for others would still be processed.
C
BGP uses UDP port 179, not TCP, so the ACL does not match BGP traffic.
Why wrong: Incorrect because BGP uses TCP port 179.
D
CoPP does not affect BGP sessions because they are established before the policy is applied.
Why wrong: Incorrect because CoPP applies to all control plane traffic, including BGP, regardless of when the session was established.

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 ACL denies all other BGP traffic, so CoPP does not match it, and it falls through to the default class, which permits it.

The correct answer is that the ACL only matches traffic from the specific peer, but CoPP class-maps match traffic based on the ACL; if the ACL denies other BGP traffic, CoPP will not match it, and it will be processed by the default class, which may permit it. Option B is incorrect because the ACL order is not the issue. Option C is incorrect because BGP uses TCP port 179, not UDP. Option D is incorrect because CoPP does not affect routing protocol sessions directly; it only polices traffic to the control plane.

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 ACL denies all other BGP traffic, so CoPP does not match it, and it falls through to the default class, which permits it.
Why this is correct
Correct because CoPP only applies to traffic matched by the class-map; if the ACL denies traffic, it is not matched, and the default class (often permit) allows it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The ACL is applied in the wrong order; the deny statement should be before the permit statement.
Why it's wrong here
Incorrect because ACL order is evaluated sequentially; a permit before deny would permit the specific peer, but the deny for others would still be processed.
✗
BGP uses UDP port 179, not TCP, so the ACL does not match BGP traffic.
Why it's wrong here
Incorrect because BGP uses TCP port 179.
✗
CoPP does not affect BGP sessions because they are established before the policy is applied.
Why it's wrong here
Incorrect because CoPP applies to all control plane traffic, including BGP, regardless of when the session was established.

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.

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 350-401 ACL questions on filtering logic and placement.

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FAQ

Questions learners often ask

What does this 350-401 question test?

ACLs and CoPP — This question tests ACLs and CoPP — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The ACL denies all other BGP traffic, so CoPP does not match it, and it falls through to the default class, which permits it. — The correct answer is that the ACL only matches traffic from the specific peer, but CoPP class-maps match traffic based on the ACL; if the ACL denies other BGP traffic, CoPP will not match it, and it will be processed by the default class, which may permit it. Option B is incorrect because the ACL order is not the issue. Option C is incorrect because BGP uses TCP port 179, not UDP. Option D is incorrect because CoPP does not affect routing protocol sessions directly; it only polices traffic to the control plane.

What should I do if I get this 350-401 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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Same concept, more angles

1 more ways this is tested on 350-401

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer is troubleshooting an issue where SSH access to a Cisco router from a specific management subnet (10.10.10.0/24) is intermittently failing. The router has a CoPP policy applied to the control plane. The engineer checks the CoPP statistics and sees that packets from the management subnet are being dropped by the control-plane service-policy. Which configuration change should the engineer make to allow SSH from the management subnet while still protecting the control plane?

medium

✓ A.Modify the CoPP ACL to include a permit statement for TCP port 22 from 10.10.10.0/24 before the deny statement.
B.Remove the deny statement from the CoPP ACL to allow all traffic.
C.Increase the police rate for the CoPP class that matches SSH traffic.
D.Remove the CoPP policy from the control plane and rely on interface ACLs.

Why A: The correct answer adds an ACL entry to permit SSH from the management subnet before the deny statement, ensuring that SSH traffic is matched by the CoPP policy and not dropped. Option B is incorrect because removing the deny statement would leave the control plane unprotected. Option C is incorrect because increasing the police rate might not resolve the issue if the traffic is being dropped by an ACL deny. Option D is incorrect because removing the CoPP policy entirely removes all protection.

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