Question 1,377 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

CoPP PIM-SM Troubleshooting — Police Rate Too Low | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of control plane policing (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 configures CoPP on a router to limit PIM-SM control plane traffic. The policy includes a class-map matching PIM packets and polices them to 10000 bps. After the policy is applied, the engineer notices that multicast traffic is not being forwarded correctly, and PIM neighbors are not forming. The router is a PIM-SM rendezvous point (RP). What is the most likely issue?

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 is dropping PIM register messages because the police rate is too low for the burst of register traffic.
PIM register messages can be large and bursty, and a police rate of 10000 bps may not be sufficient, causing drops and preventing RP functionality.
B
The CoPP class-map is not matching PIM packets because it uses the wrong protocol number.
Why wrong: PIM uses IP protocol 103; if the class-map matches correctly, this is not the issue.
C
The PIM hello interval is set too high, causing the router to miss hello packets from neighbors.
Why wrong: A higher hello interval would reduce traffic, not cause drops.
D
The CoPP policy is applied to the wrong control plane, such as the IPv6 control plane.
Why wrong: CoPP is applied per control plane (IPv4 or IPv6), but the scenario implies IPv4 PIM.

Full breakdown with real-world context →

Quick Answer

The answer is that the CoPP police rate is too low for the bursty nature of PIM-SM register messages, causing them to be dropped. When a router acts as a PIM-SM rendezvous point, it must handle register traffic from designated routers, which can arrive in large bursts as sources begin sending. If the CoPP policy limits PIM control plane traffic to only 10000 bps, these bursts exceed the police rate, leading to packet drops that prevent PIM neighbor formation and disrupt multicast forwarding. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how CoPP interacts with PIM-SM operations, often appearing as a trick where candidates overlook the difference between steady-state hello messages and bursty register traffic. A common trap is assuming a low police rate is sufficient for all PIM packets, but register messages are larger and more frequent during initial source registration. Memory tip: "Register bursts need bigger bursts"—always size CoPP for the peak, not the average.

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 dropping PIM register messages because the police rate is too low for the burst of register traffic.

When a router is configured as a PIM-SM rendezvous point (RP), it receives PIM register messages from first-hop routers. These messages are encapsulated unicast packets that can arrive in bursts, especially when multiple sources register simultaneously. If the CoPP police rate is set too low (10000 bps), the router may drop these register messages, preventing the RP from learning about new multicast sources and causing PIM neighbors to fail to form.

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 is dropping PIM register messages because the police rate is too low for the burst of register traffic.
Why this is correct
PIM register messages can be large and bursty, and a police rate of 10000 bps may not be sufficient, causing drops and preventing RP functionality.
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.
✗
The CoPP class-map is not matching PIM packets because it uses the wrong protocol number.
Why it's wrong here
PIM uses IP protocol 103; if the class-map matches correctly, this is not the issue.
✗
The PIM hello interval is set too high, causing the router to miss hello packets from neighbors.
Why it's wrong here
A higher hello interval would reduce traffic, not cause drops.
✗
The CoPP policy is applied to the wrong control plane, such as the IPv6 control plane.
Why it's wrong here
CoPP is applied per control plane (IPv4 or IPv6), but the scenario implies IPv4 PIM.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that CoPP only affects routing protocol hellos or keepalives, when in fact it can drop critical control messages like PIM register messages that are essential for RP operation in PIM-SM.

Trap categories for this question

Scenario analysis trap
CoPP is applied per control plane (IPv4 or IPv6), but the scenario implies IPv4 PIM.

Detailed technical explanation

How to think about this question

PIM register messages are sent as unicast packets from the designated router (DR) to the RP, encapsulated in a PIM register header. The RP must decapsulate these to create (S,G) state and send join messages toward the source. If CoPP drops these registers, the RP never builds the multicast tree, and traffic fails. A typical CoPP policy for PIM should allow for bursty register traffic, often by setting a conform-action 'transmit' and exceed-action 'drop' with a reasonable burst size (e.g., 1500 bytes per packet).

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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

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?

Control Plane Policing (CoPP) — This question tests Control Plane Policing (CoPP) — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The CoPP policy is dropping PIM register messages because the police rate is too low for the burst of register traffic. — When a router is configured as a PIM-SM rendezvous point (RP), it receives PIM register messages from first-hop routers. These messages are encapsulated unicast packets that can arrive in bursts, especially when multiple sources register simultaneously. If the CoPP police rate is set too low (10000 bps), the router may drop these register messages, preventing the RP from learning about new multicast sources and causing PIM neighbors to fail to form.

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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