- A
The BFD multiplier on R1 is too low.
Why wrong: Incorrect. The BFD multiplier of 3 on R1 is standard and not too low. The multiplier affects detection time, but with negotiated intervals of 200 ms, the detection time is 600 ms, which is reasonable.
- B
R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.
Incorrect. BFD timers negotiate so that the actual transmit interval is the maximum of the local min_tx and the remote min_rx. Both devices end up with a transmit interval of 200 ms, so session establishment should occur. The mismatch does not cause failure.
- C
EIGRP must be configured with 'bfd all-interfaces' to work.
Why wrong: Incorrect. The 'bfd all-interfaces' command is not required for EIGRP BFD; BFD is enabled per interface or neighbor. This option is unrelated.
- D
The serial link requires 'no ip split-horizon' for BFD.
Why wrong: Incorrect. Split horizon is a routing protocol feature and has no impact on BFD session establishment.
BFD Timer Mismatch
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. A key principle to apply: bFD timer negotiation. 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?
Quick Answer
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.”
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
R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.
The BFD timers are negotiated symmetrically: R1's min_rx of 50 ms and R2's min_tx of 200 ms result in both transmitting at 200 ms intervals, with detection times of 600 ms each. This timer mismatch does not prevent session establishment; the session should come up. The observed state (R1 shows BFD pending, R2 shows BFD up) indicates a unidirectional issue, such as an ACL blocking BFD packets from R2 to R1. None of the provided options correctly describe the root cause.
Key principle: BFD timer negotiation
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
Incorrect. The BFD multiplier of 3 on R1 is standard and not too low. The multiplier affects detection time, but with negotiated intervals of 200 ms, the detection time is 600 ms, which is reasonable.
- ✓
R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.
Why this is correct
Incorrect. BFD timers negotiate so that the actual transmit interval is the maximum of the local min_tx and the remote min_rx. Both devices end up with a transmit interval of 200 ms, so session establishment should occur. The mismatch does not cause failure.
Related concept
BFD timer negotiation
- ✗
EIGRP must be configured with 'bfd all-interfaces' to work.
Why it's wrong here
Incorrect. The 'bfd all-interfaces' command is not required for EIGRP BFD; BFD is enabled per interface or neighbor. This option is unrelated.
- ✗
The serial link requires 'no ip split-horizon' for BFD.
Why it's wrong here
Incorrect. Split horizon is a routing protocol feature and has no impact on BFD session establishment.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
Incorrect. The 'bfd all-interfaces' command is not required for EIGRP BFD; BFD is enabled per interface or neighbor. This option is unrelated.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- BFD timer negotiation
- min_rx and min_tx intervals
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
BFD timer negotiation
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
Got this wrong? Here's your next step.
Review bFD timer negotiation, then practise related 300-410 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — BFD timer negotiation.
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. — The BFD timers are negotiated symmetrically: R1's min_rx of 50 ms and R2's min_tx of 200 ms result in both transmitting at 200 ms intervals, with detection times of 600 ms each. This timer mismatch does not prevent session establishment; the session should come up. The observed state (R1 shows BFD pending, R2 shows BFD up) indicates a unidirectional issue, such as an ACL blocking BFD packets from R2 to R1. None of the provided options correctly describe the root cause.
What should I do if I get this 300-410 question wrong?
Review bFD timer negotiation, then practise related 300-410 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
BFD timer negotiation
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Last reviewed: Jun 18, 2026
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