The answer is a BFD timer mismatch where R1’s min_rx interval of 50 ms is lower than R2’s min_tx interval of 200 ms, causing the BFD session to fail on R1 while appearing up on R2. This occurs because BFD timers are negotiated asymmetrically: each router’s min_rx dictates the minimum interval it can receive, and the neighbor’s min_tx must be equal to or greater than that value. Here, R2’s configured min_tx of 200 ms cannot satisfy R1’s demand to receive at 50 ms, so R2 rejects the session, leaving R1’s EIGRP neighbor in a ‘Pending’ BFD state. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BFD session establishment is prerequisite to EIGRP adjacency, and a common trap is assuming both sides must match—they don’t; the critical check is that each side’s min_rx must be <= the other side’s min_tx. Memory tip: “RX must be less than or equal to the neighbor’s TX—if your RX is too fast, the session won’t last.”
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). 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.
R1 and R2 are EIGRP neighbors over a serial link with BFD enabled. R1#show ip eigrp neighbors shows R2 in state 'Pending' for BFD. R2#show bfd neighbors shows the session as 'Up'. R1 has 'bfd interval 50 min_rx 50 multiplier 3'. R2 has 'bfd interval 200 min_rx 200 multiplier 3'. 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
✓
R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.
EIGRP BFD requires that the BFD session be established before EIGRP can form adjacency. If the BFD session is up on one side but not the other, it indicates a timer mismatch. However, BFD timers are negotiated; the issue is that R1's min_rx of 50 ms is lower than R2's min_rx of 200 ms, causing R2 to reject the session because it cannot transmit at that rate.
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 BFD multiplier on R1 is too low.
Why it's wrong here
Multiplier 3 is standard.
✓
R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.
Why this is correct
R1's min_rx of 50 ms is less than R2's min_tx of 200 ms, so R2 cannot meet the required receive rate, causing BFD session to fail.
EIGRP must be configured with 'bfd all-interfaces' to work.
Why it's wrong here
BFD is enabled per interface.
✗
The serial link requires 'no ip split-horizon' for BFD.
Why it's wrong here
Split horizon does not affect BFD.
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.
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure. — EIGRP BFD requires that the BFD session be established before EIGRP can form adjacency. If the BFD session is up on one side but not the other, it indicates a timer mismatch. However, BFD timers are negotiated; the issue is that R1's min_rx of 50 ms is lower than R2's min_rx of 200 ms, causing R2 to reject the session because it cannot transmit at that rate.
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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