hardMultiple Choice
300-410 Practice Question: A router has CoPP configured with a class-map…
A router has CoPP configured with a class-map that matches OSPF traffic and polices it to 2000 pps. The router is also configured with an OSPF distribute-list in to filter routes. After applying CoPP, OSPF neighbors form, but routes from a specific neighbor are missing. The distribute-list permits all routes. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the distinction between OSPF neighbor formation (Hello packets) and route exchange (LSU packets), leading candidates to assume that if neighbors are up, all OSPF traffic is passing correctly.
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
✓
CoPP drops OSPF LSU packets, preventing route installation, while hello packets still form the adjacency.
OSPF uses Hello packets to establish and maintain neighbor adjacencies, but route information is exchanged via Link State Update (LSU) packets. The CoPP policy matches OSPF traffic and polices it to 2000 pps, which can drop LSU packets if the rate is exceeded, preventing route installation while allowing enough Hello packets to keep the adjacency up. The distribute-list permits all routes, so it is not the cause of the missing routes.
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 distribute-list is applied incorrectly and blocks all routes.
Why it's wrong here
The stem states the distribute-list permits all routes, so it cannot be the cause of the missing routes. It is tempting because distribute-lists commonly filter OSPF routes, and an incorrectly applied one would be the right answer when neighbours form but prefixes are absent without a stated permit-all entry.
- ✓
CoPP drops OSPF LSU packets, preventing route installation, while hello packets still form the adjacency.
Why this is correct
CoPP polices matched OSPF traffic at 2000 pps; hello packets are small and infrequent so the adjacency still forms, but larger LSU packets exceed the policer and get dropped. Without LSAs, the router never installs those routes, and the permissive distribute-list is irrelevant.
- ✗
OSPF uses TCP, and CoPP only polices UDP.
Why it's wrong here
OSPF runs directly over IP protocol 89, not TCP or UDP, so the premise is false and cannot explain the loss. It is tempting because CoPP class-maps often match UDP or TCP ports, and protocol mismatch would be the answer if OSPF genuinely used a transport protocol that the policy dropped.
- ✗
The CoPP policy is applied to the wrong control plane subinterface.
Why it's wrong here
CoPP polices traffic punted to the route processor; it is not applied to a control plane subinterface, so this mechanism does not exist as described. It is tempting because CoPP and control plane policing terminology overlap, and subinterface-level policy would be relevant when shaping per-interface transit traffic rather than protecting the RP.
Visual reference
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
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