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300-410 Practice Question: Configures CoPP on a router to limit PIM-SM…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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