Question 1,377 of 2,152

Control Plane Policing (CoPP) →hardMultiple ChoiceObjective-mapped

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.

300-410 Control Plane Policing (CoPP) Practice Question

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.

Question 1hardmultiple choice

Study the full ACL explanation →

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 →

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.

PIM-SM uses periodic hello messages and register messages that can be large. If the police rate is too low, PIM packets are dropped, preventing neighbor formation and RP discovery. Additionally, the RP might need to process register messages, which can be bursty.

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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
OSPF neighbours must agree on key parameters.
✗
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: OSPF can fail even when IP connectivity looks correct

OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.

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

OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

Key takeaway

OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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.

Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.

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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) — OSPF neighbours must agree on key parameters..

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. — PIM-SM uses periodic hello messages and register messages that can be large. If the police rate is too low, PIM packets are dropped, preventing neighbor formation and RP discovery. Additionally, the RP might need to process register messages, which can be bursty.

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?

OSPF neighbours must agree on key parameters.

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