- A
The flow monitor is applied to the tunnel interface in the input direction and uses a flow record that matches on BGP port 179, causing BGP packets to be dropped.
Why wrong: Flexible NetFlow does not drop packets by default; it only monitors them.
- B
The flow exporter's source interface is set to Tunnel0, but the tunnel is not yet established, so BGP updates sourced from Tunnel0 are unreachable.
If the exporter source is the tunnel interface, the router will try to send BGP packets with that source IP, but the tunnel is not up, so the packets are dropped. This prevents BGP from establishing.
- C
The BGP neighbor is configured with a password that does not match the hub.
Why wrong: The BGP neighbor is up, so authentication is not the issue.
- D
The NHRP authentication key is missing, preventing spoke-to-spoke communication.
Why wrong: The show output indicates NHRP state is present, so NHRP is working.
Quick Answer
The answer is a misconfigured flow exporter source interface on Tunnel0, which prevents BGP updates from reaching the hub. This creates a chicken-and-egg problem: the Flexible NetFlow exporter is set to source from Tunnel0, but the tunnel cannot establish until BGP learns the remote spoke’s routes, and BGP cannot learn those routes because its updates are sourced from an unreachable tunnel IP. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how Flexible NetFlow interacts with DMVPN control plane traffic—a common trap is assuming the flow monitor itself drops packets, when the real culprit is the exporter’s source interface referencing a tunnel that isn’t yet up. Remember: if BGP neighbors are up but no routes are received, check the source of your updates, not just the neighbor state. Memory tip: “Source before tunnel—if the exporter points to a tunnel that’s down, BGP routes won’t come around.”
300-410 NetFlow and Flexible NetFlow Practice Question
This 300-410 practice question tests your understanding of netflow and flexible netflow. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A DMVPN network uses FlexVPN with BGP as the routing protocol. Spoke routers are configured with Flexible NetFlow to monitor traffic. After a configuration change, spoke-to-spoke tunnels fail to establish. Router R1 (spoke) shows: show dmvpn detail | include (State|Tunnel) State: NHRP, Tunnel: Tunnel0. The BGP neighbor to the hub is up, but no BGP routes are received for the remote spoke's LAN. 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 exporter's source interface is set to Tunnel0, but the tunnel is not yet established, so BGP updates sourced from Tunnel0 are unreachable.
Flexible NetFlow can be applied to the tunnel interface, but if the flow monitor is configured with a sampler or a flow record that changes the interface behavior, it might interfere with NHRP or BGP. Specifically, if the flow monitor is configured to use a flow record that includes the 'input interface' field and is applied in the input direction, it may cause the router to process packets differently, potentially dropping NHRP packets or BGP updates. However, the most likely issue is that the flow monitor is configured with a match statement that inadvertently matches BGP packets and applies an action (like drop) or that the flow exporter is misconfigured to use the tunnel interface as source, causing BGP updates to be sourced from the tunnel IP, which is not reachable. The correct answer is that the flow exporter's source interface is set to Tunnel0, which is not reachable from the hub until the tunnel is up, creating a chicken-and-egg problem.
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 applied to the tunnel interface in the input direction and uses a flow record that matches on BGP port 179, causing BGP packets to be dropped.
Why it's wrong here
Flexible NetFlow does not drop packets by default; it only monitors them.
- ✓
The flow exporter's source interface is set to Tunnel0, but the tunnel is not yet established, so BGP updates sourced from Tunnel0 are unreachable.
- ✗
The BGP neighbor is configured with a password that does not match the hub.
Why it's wrong here
The BGP neighbor is up, so authentication is not the issue.
- ✗
The NHRP authentication key is missing, preventing spoke-to-spoke communication.
Why it's wrong here
The show output indicates NHRP state is present, so NHRP is working.
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.
Trap categories for this question
Command / output trap
The show output indicates NHRP state is present, so NHRP is working.
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 exporter's source interface is set to Tunnel0, but the tunnel is not yet established, so BGP updates sourced from Tunnel0 are unreachable. — Flexible NetFlow can be applied to the tunnel interface, but if the flow monitor is configured with a sampler or a flow record that changes the interface behavior, it might interfere with NHRP or BGP. Specifically, if the flow monitor is configured to use a flow record that includes the 'input interface' field and is applied in the input direction, it may cause the router to process packets differently, potentially dropping NHRP packets or BGP updates. However, the most likely issue is that the flow monitor is configured with a match statement that inadvertently matches BGP packets and applies an action (like drop) or that the flow exporter is misconfigured to use the tunnel interface as source, causing BGP updates to be sourced from the tunnel IP, which is not reachable. The correct answer is that the flow exporter's source interface is set to Tunnel0, which is not reachable from the hub until the tunnel is up, creating a chicken-and-egg problem.
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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