The answer is that a flow monitor using a sampler configured to sample only 1 out of 100 packets causes LDP hello messages to be missed, preventing the session from advancing past the INIT state. This occurs because Flexible NetFlow applied to the core-facing interface intercepts packets before they reach the LDP process; when a sampler is active, it selectively drops the majority of packets, including the periodic LDP hello messages required to maintain the transport connection. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how Flexible NetFlow can interfere with control plane protocols—a common trap is assuming the issue is CPU overload rather than packet sampling. The key clue is the asymmetric neighbor states: one router shows OPERATIONAL while the other is stuck in INIT, indicating unidirectional hello loss. Memory tip: “Sampled hellos, session yellow” — if LDP stays INIT, check for a sampler on the interface.
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 MPLS network uses LDP for label distribution. After enabling Flexible NetFlow on the core routers, some LDP sessions fail to establish. Router R1 shows: show mpls ldp neighbor | include (Peer|State) Peer LDP Ident: 10.0.0.2:0, State: OPERATIONAL. Router R2 shows: show mpls ldp neighbor | include (Peer|State) Peer LDP Ident: 10.0.0.1:0, State: INIT. What is the root cause?
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 LDP hello messages to be missed.
Flexible NetFlow can consume CPU resources or change the way packets are processed. In this case, the LDP session is stuck in INIT state, which typically indicates a problem with the transport connection or hello messages. If a flow monitor is applied to the interface used for LDP (e.g., the core-facing interface), it might be using a sampler that samples only a fraction of packets, causing LDP hello messages to be missed. Alternatively, the flow monitor might be configured to use a flow record that includes the 'ipv4 ttl' field, which could cause the router to process LDP packets differently. The correct answer is that a flow sampler is configured on the interface, causing LDP hello packets to be sampled and potentially dropped, preventing the LDP session from moving to OPERATIONAL.
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 LDP hello messages to be missed.
Why this is correct
LDP hellos are sent every 5 seconds; if they are sampled out, the neighbor will not receive them, and the session will stay in INIT.
The LDP router-id is misconfigured, causing a conflict with the flow exporter.
Why it's wrong here
The LDP router-id is separate from flow exporter configuration.
✗
The MPLS MTU is set too low, causing LDP packets to be fragmented.
Why it's wrong here
Fragmentation would not cause INIT state; it would cause other issues.
✗
The flow exporter is configured to use TCP port 646, conflicting with LDP.
Why it's wrong here
LDP uses UDP and TCP port 646; the exporter uses UDP 2055 typically, so no conflict.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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 LDP hello messages to be missed. — Flexible NetFlow can consume CPU resources or change the way packets are processed. In this case, the LDP session is stuck in INIT state, which typically indicates a problem with the transport connection or hello messages. If a flow monitor is applied to the interface used for LDP (e.g., the core-facing interface), it might be using a sampler that samples only a fraction of packets, causing LDP hello messages to be missed. Alternatively, the flow monitor might be configured to use a flow record that includes the 'ipv4 ttl' field, which could cause the router to process LDP packets differently. The correct answer is that a flow sampler is configured on the interface, causing LDP hello packets to be sampled and potentially dropped, preventing the LDP session from moving to OPERATIONAL.
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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