- A
CoPP drops routing updates, causing the routing table to lack the neighbor's source network, so uRPF drops the neighbor's packets.
uRPF strict mode requires a route back to the source; without it, packets are dropped, including routing updates.
- B
uRPF is applied after CoPP, so CoPP drops the routing updates first.
Why wrong: uRPF is applied at the interface level before CoPP.
- C
The CoPP policy matches all IP traffic, including uRPF failure packets.
Why wrong: uRPF drops packets before they reach the control plane.
- D
Pings succeed because they use a different protocol than routing updates.
Why wrong: Pings use ICMP, which is also subject to uRPF and CoPP.
Question 1,413 of 2,152
How CoPP and uRPF Cause a Feedback Loop Dropping Routing Updates
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. A key principle to apply: control Plane Policing (CoPP). 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.
An engineer configures CoPP with a class-map that matches all IP traffic and polices it to 10000 pps. The router also has uRPF strict mode enabled on the WAN interface. After applying CoPP, the router stops receiving routing updates from a neighbor, but pings to the neighbor succeed. Which is the most likely explanation?
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.
Quick Answer
The answer is that CoPP drops routing updates, which causes the routing table to lack the neighbor’s source network, so uRPF strict mode then drops the neighbor’s packets. This creates a feedback loop because uRPF strict mode routing drops occur before CoPP processes traffic—uRPF checks the Forwarding Information Base (FIB) for source reachability, and if CoPP has already throttled routing protocol packets, the FIB becomes incomplete, causing uRPF to discard valid packets from the neighbor. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of packet flow order: uRPF operates on the ingress interface before control-plane policing, so a missing route from CoPP starvation directly triggers uRPF drops, even though pings succeed (since ping replies use a different path or are not rate-limited). A common trap is assuming CoPP alone is the culprit, but the real issue is the dependency loop. Memory tip: “CoPP starves the FIB, uRPF kills the feed.”
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 routing updates, causing the routing table to lack the neighbor's source network, so uRPF drops the neighbor's packets.
CoPP is applied to control plane traffic, but it does not process packets before uRPF on the ingress interface. In this scenario, the class-map matching all IP traffic and policing to 10000 pps drops routing update packets (e.g., OSPF, EIGRP, BGP) when they exceed the policer rate. Without these updates, the router lacks a route to the neighbor's source network. Therefore, when the neighbor sends packets to the router (e.g., pings from neighbor to router), uRPF strict mode on the WAN interface drops them because the reverse path check fails. Pings from the router to the neighbor succeed because they are outgoing and not subject to inbound uRPF. Thus, CoPP indirectly causes uRPF drops, leading to the observed behavior.
Key principle: Control Plane Policing (CoPP)
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
CoPP drops routing updates, causing the routing table to lack the neighbor's source network, so uRPF drops the neighbor's packets.
Why this is correct
uRPF strict mode requires a route back to the source; without it, packets are dropped, including routing updates.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Control Plane Policing (CoPP)
- ✗
uRPF is applied after CoPP, so CoPP drops the routing updates first.
Why it's wrong here
uRPF is applied at the interface level before CoPP.
- ✗
The CoPP policy matches all IP traffic, including uRPF failure packets.
Why it's wrong here
uRPF drops packets before they reach the control plane.
- ✗
Pings succeed because they use a different protocol than routing updates.
Why it's wrong here
Pings use ICMP, which is also subject to uRPF and CoPP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The key trap is that CoPP dropping routing updates causes loss of routing table entries, which then triggers uRPF to drop packets from that neighbor. Candidates may overlook the causal chain and incorrectly believe uRPF drops before CoPP, missing that CoPP indirectly causes uRPF failures.
Detailed technical explanation
How to think about this question
uRPF strict mode performs a reverse path lookup on the incoming interface; if the source IP address is not reachable via that interface in the FIB, the packet is dropped. CoPP uses a control-plane service policy to rate-limit packets destined to the router itself, such as routing protocol packets. In this scenario, the routing protocol packets (e.g., OSPF hello packets on UDP 89) are policed to 10000 pps, and if the neighbor sends bursts exceeding that rate, CoPP drops them, causing the routing adjacency to fail. The FIB then lacks the neighbor's source network, so uRPF drops subsequent packets from that neighbor, including pings if they arrive at a high rate.
KKey Concepts to Remember
- Control Plane Policing (CoPP)
- Unicast Reverse Path Forwarding (uRPF)
- Order of Operations
- Routing Update Drops
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
Control Plane Policing (CoPP)
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.
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.
What to study next
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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) — Control Plane Policing (CoPP).
What is the correct answer to this question?
The correct answer is: CoPP drops routing updates, causing the routing table to lack the neighbor's source network, so uRPF drops the neighbor's packets. — CoPP is applied to control plane traffic, but it does not process packets before uRPF on the ingress interface. In this scenario, the class-map matching all IP traffic and policing to 10000 pps drops routing update packets (e.g., OSPF, EIGRP, BGP) when they exceed the policer rate. Without these updates, the router lacks a route to the neighbor's source network. Therefore, when the neighbor sends packets to the router (e.g., pings from neighbor to router), uRPF strict mode on the WAN interface drops them because the reverse path check fails. Pings from the router to the neighbor succeed because they are outgoing and not subject to inbound uRPF. Thus, CoPP indirectly causes uRPF drops, leading to the observed behavior.
What should I do if I get this 300-410 question wrong?
Review control Plane Policing (CoPP), then practise related 300-410 questions on the same topic to reinforce the concept.
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?
Control Plane Policing (CoPP)
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Last reviewed: Jul 4, 2026
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