- A
The flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed.
OSPF hellos are sent every 10 seconds; if they are sampled out, the neighbor will not receive them, and the neighbor state will go down.
- B
The flow monitor is configured with a flow record that includes the 'ipv4 ttl' field, causing the router to decrement the TTL of OSPF packets, making them invalid.
Why wrong: Flexible NetFlow does not modify packets; it only monitors them.
- C
The flow exporter is configured to use the OSPF router ID as the source, causing a conflict.
Why wrong: The exporter source is an IP address, not a router ID.
- D
The OSPF network type is misconfigured on the interface, but the flow monitor is not related.
Why wrong: The question states the flow monitor was applied after the configuration, so the network type is not the issue.
Quick Answer
The correct answer is that the flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed. Flexible NetFlow samplers are designed to select a subset of packets for monitoring, and when applied to an interface, they can inadvertently drop or ignore control-plane packets like OSPF hellos, which are sent every 10 seconds by default. If the sampler rate is too aggressive—such as 1:100—the router may fail to process enough hello packets to maintain the neighbor adjacency, leading to the DOWN state even though the flow monitor itself reports no errors. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how NetFlow features interact with routing protocols, often appearing as a trap where candidates blame packet corruption or misconfiguration of OSPF timers. A key memory tip: if OSPF neighbors drop after applying a flow monitor, always check the sampler configuration first—think “sampler skips hellos, neighbors go silent.”
300-410 NetFlow and Flexible NetFlow Practice Question
This 300-410 practice question tests your understanding of netflow and flexible netflow. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
An OSPF network has multiple areas and uses Flexible NetFlow to monitor inter-area traffic. After applying a flow monitor to the ABR's interface, OSPF neighbor relationships fail to form. Router R1 (ABR) shows: show ip ospf neighbor | include (FULL|DOWN) Neighbor 10.0.0.2, interface GigabitEthernet0/0, state DOWN. show flow monitor FLOW-MONITOR statistics | include (Packets|Errors) Packets exported: 1000, Errors: 0. What is the root cause?
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 flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed.
OSPF neighbor relationships can fail if the interface is configured with a flow monitor that uses a sampler or if the flow record includes fields that cause the router to process OSPF packets incorrectly. In this case, the flow monitor is exporting packets successfully, but the OSPF neighbor is down. The most likely cause is that the flow monitor is configured with a flow record that includes the 'ipv4 ttl' field, and the router is using that to modify the TTL of OSPF packets, causing them to be dropped. However, the correct answer is that the flow monitor is applied in the output direction and is using a flow record that includes the 'ipv4 dscp' field, but the OSPF packets have a specific DSCP value that is being changed by the flow monitor, causing the neighbor to reject them. Actually, the most plausible root cause is that the flow monitor is configured with a match statement that matches on OSPF packets (protocol 89) and applies an action to drop them, but Flexible NetFlow does not drop packets. The correct answer is that the flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed, leading to neighbor down.
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 flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed.
- ✗
The flow monitor is configured with a flow record that includes the 'ipv4 ttl' field, causing the router to decrement the TTL of OSPF packets, making them invalid.
Why it's wrong here
Flexible NetFlow does not modify packets; it only monitors them.
- ✗
The flow exporter is configured to use the OSPF router ID as the source, causing a conflict.
Why it's wrong here
The exporter source is an IP address, not a router ID.
- ✗
The OSPF network type is misconfigured on the interface, but the flow monitor is not related.
Why it's wrong here
The question states the flow monitor was applied after the configuration, so the network type is not the issue.
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.
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?
NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed. — OSPF neighbor relationships can fail if the interface is configured with a flow monitor that uses a sampler or if the flow record includes fields that cause the router to process OSPF packets incorrectly. In this case, the flow monitor is exporting packets successfully, but the OSPF neighbor is down. The most likely cause is that the flow monitor is configured with a flow record that includes the 'ipv4 ttl' field, and the router is using that to modify the TTL of OSPF packets, causing them to be dropped. However, the correct answer is that the flow monitor is applied in the output direction and is using a flow record that includes the 'ipv4 dscp' field, but the OSPF packets have a specific DSCP value that is being changed by the flow monitor, causing the neighbor to reject them. Actually, the most plausible root cause is that the flow monitor is configured with a match statement that matches on OSPF packets (protocol 89) and applies an action to drop them, but Flexible NetFlow does not drop packets. The correct answer is that the flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed, leading to neighbor down.
What should I do if I get this 300-410 question wrong?
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 19, 2026
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