Question 198 of 2,152

Route Maps and Route Filtering →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol. This is the root cause because BGP uses TCP port 179 for its session, not UDP; the misconfigured access-list 100 permits UDP traffic, meaning the CoPP policy never actually polices BGP packets, and the drops shown on R1 are from some other UDP-based protocol hitting the same class. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to correlate CoPP troubleshooting with BGP flapping—a common trap is to immediately assume the police rate is too low, but the real issue is often a mismatched protocol in the ACL. Remember that BGP is a TCP-based routing protocol, so any CoPP ACL targeting it must specify TCP, not UDP. A quick memory tip: “BGP rides TCP, so CoPP must match TCP 179—UDP drops are a red herring.”

300-410 Route Maps and Route Filtering Practice Question

This 300-410 practice question tests your understanding of route maps and route filtering. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 enterprise is using CoPP to protect the control plane. R1 has the following configuration: access-list 100 permit udp any any eq 179 class-map match-any BGP match access-group 100 policy-map COPP class BGP police 100000 20000 conform-action transmit exceed-action drop. Router R2 shows: 'show ip bgp summary' indicates the BGP session to R1 is flapping every 30 seconds. R1's 'show policy-map control-plane' shows drops for class BGP. What is the root cause?

Question 1hardmultiple choice

Open the full BGP breakdown →

A
The CoPP policy is rate-limiting BGP traffic too aggressively, causing BGP packets to be dropped and the session to flap.
BGP uses TCP port 179, and the ACL matches all UDP traffic to port 179, but BGP uses TCP, not UDP. However, the ACL incorrectly uses UDP, so it does not match BGP traffic at all. The drops are likely from another class, or the session flapping is due to another reason. Wait, the ACL uses UDP, so it does not match BGP (TCP). The correct answer should be that the ACL is misconfigured.
B
The ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol.
BGP uses TCP port 179, not UDP. The ACL 'permit udp any any eq 179' does not match BGP packets, so the CoPP policy does not rate-limit BGP. The session flapping must be due to another cause, such as a different CoPP class or network issue. However, the question states drops for class BGP, so the class-map must be matching something else. The correct root cause is that the ACL is misconfigured, and the drops are for UDP traffic that happens to use port 179 (e.g., some other application).
C
The police rate is set too high, causing the router to drop all BGP traffic.
Why wrong: A high police rate would not cause drops; it would allow traffic.
D
The class-map is not applied to the control-plane, so the policy has no effect.
Why wrong: The policy-map is applied to the control-plane, as shown in the show command.

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 CoPP policy is rate-limiting BGP traffic too aggressively, causing BGP packets to be dropped and the session to flap.

CoPP is rate-limiting BGP traffic (TCP port 179) to 100 kbps with a burst of 20 kbps. BGP keepalives and updates can exceed this rate, especially if there are many prefixes or if the session is flapping. The drops cause BGP packets to be lost, leading to hold timer expiry and session flapping. The root cause is that the police rate is too low for the BGP traffic volume. The correct fix is to increase the police rate or add a more specific match to only rate-limit certain BGP packets.

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 CoPP policy is rate-limiting BGP traffic too aggressively, causing BGP packets to be dropped and the session to flap.
Why this is correct
BGP uses TCP port 179, and the ACL matches all UDP traffic to port 179, but BGP uses TCP, not UDP. However, the ACL incorrectly uses UDP, so it does not match BGP traffic at all. The drops are likely from another class, or the session flapping is due to another reason. Wait, the ACL uses UDP, so it does not match BGP (TCP). The correct answer should be that the ACL is misconfigured.
Related concept
Standard ACLs match source addresses.
✓
The ACL in the class-map uses UDP instead of TCP, so it does not match BGP traffic, and the drops are from another protocol.
Why this is correct
BGP uses TCP port 179, not UDP. The ACL 'permit udp any any eq 179' does not match BGP packets, so the CoPP policy does not rate-limit BGP. The session flapping must be due to another cause, such as a different CoPP class or network issue. However, the question states drops for class BGP, so the class-map must be matching something else. The correct root cause is that the ACL is misconfigured, and the drops are for UDP traffic that happens to use port 179 (e.g., some other application).
Related concept
Standard ACLs match source addresses.
✗
The police rate is set too high, causing the router to drop all BGP traffic.
Why it's wrong here
A high police rate would not cause drops; it would allow traffic.
✗
The class-map is not applied to the control-plane, so the policy has no effect.
Why it's wrong here
The policy-map is applied to the control-plane, as shown in the show command.

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
The policy-map is applied to the control-plane, as shown in the show command.

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?

Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The CoPP policy is rate-limiting BGP traffic too aggressively, causing BGP packets to be dropped and the session to flap. — CoPP is rate-limiting BGP traffic (TCP port 179) to 100 kbps with a burst of 20 kbps. BGP keepalives and updates can exceed this rate, especially if there are many prefixes or if the session is flapping. The drops cause BGP packets to be lost, leading to hold timer expiry and session flapping. The root cause is that the police rate is too low for the BGP traffic volume. The correct fix is to increase the police rate or add a more specific match to only rate-limit certain BGP packets.

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

What is the key concept behind this question?

Standard ACLs match source addresses.

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