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300-410 Practice Question: A network uses Control Plane Policing (CoPP) to…
A network uses Control Plane Policing (CoPP) to protect the router. Router R1 has CoPP policy applied that rate-limits all traffic to 1 Mbps. R1 shows: 'show policy-map control-plane' indicates drops for EIGRP packets. R1's EIGRP neighbor R2 is flapping. What is the root cause?
⚠ Common exam trap
Cisco often tests the misconception that CoPP drops are caused by exceeding the bandwidth rate-limit, when in reality the trap is that small, frequent control-plane packets (like EIGRP hellos) are dropped due to an insufficient burst size, not the overall traffic rate.
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 rate-limits EIGRP packets to 1 Mbps, but EIGRP hellos are small and frequent; the rate-limit may still drop them if the policer is not properly configured for burst.
EIGRP hello packets are small and sent frequently (every 5 seconds on most LAN interfaces by default). A CoPP policer that rate-limits all traffic to 1 Mbps without an adequate burst parameter will treat the burst of small hello packets as exceeding the committed burst (Bc), causing drops. These drops prevent EIGRP neighbor keepalives from reaching the control plane, leading to neighbor flapping. The root cause is not the total bandwidth but the policer's inability to accommodate the packet-per-second (PPS) burst of small EIGRP hellos.
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 rate-limits EIGRP packets to 1 Mbps, but EIGRP hellos are small and frequent; the rate-limit may still drop them if the policer is not properly configured for burst.
Why this is correct
Even small packets can be dropped if the policer uses a strict rate without adequate burst size.
- ✗
R2 is sending too many EIGRP packets, exceeding the rate-limit.
Why it's wrong here
The issue is the policer, not excessive traffic.
- ✗
R1 has an ACL that blocks EIGRP packets before they reach the control plane.
Why it's wrong here
CoPP is applied to the control plane, not an ACL.
- ✗
The CoPP policy is applied to the wrong interface.
Why it's wrong here
CoPP operates on the control plane itself, not on a specific interface; the policy is applied globally to the control plane via the `control-plane` configuration mode, so the interface is irrelevant to the drops. This option is tempting because interface-based policies like ACLs or CoPP's predecessor, Control Plane Protection (CPPr), do apply per interface, and in a scenario where traffic bypasses the control plane due to misapplied interface ACLs, an interface-level fix would be correct.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.