Quick Answer
The answer is an ACL blocking UDP port 3784 from R2 on R1. This is correct because BFD sessions rely on bidirectional UDP hello packets on port 3784, and when R1’s inbound ACL silently drops these packets from R2, R1 never receives the BFD control messages needed to bring the session up—even though R2 sees R1’s BFD hellos and reports the session as up. This asymmetric state, where one neighbor shows the BFD session down while the other shows it up, is a classic symptom of a unidirectional firewall or ACL filter. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to distinguish between BFD session failures caused by reachability issues versus configuration mismatches—a common trap is to blame the mismatch in BFD timers (50/50 vs 100/100), but those are compatible as long as the minimum intervals are met. Memory tip: “BFD is a two-way street—if only one side sees the other, check for a silent ACL on port 3784.”
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 IS-IS neighbors with BFD enabled. R1#show clns is-neighbors shows R2 as 'Up'. R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'bfd interval 50 min_rx 50 multiplier 3' on the interface. R2 has 'bfd interval 100 min_rx 100 multiplier 3'. The link is stable. What is the root cause?
R1 has an ACL blocking UDP port 3784 from R2.
BFD uses UDP port 3784; if R1 blocks incoming BFD packets, the session appears down on R1 but up on R2 because R2 receives R1's packets.
IS-IS requires 'bfd all-interfaces' to work with BFD.
Why wrong: Per-interface BFD is supported.
The BFD multiplier on R1 is too low.
Why wrong: Multiplier 3 is standard.
The IS-IS metric must be set to 1 for BFD.
Why wrong: Metric does not affect BFD.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
R1 has an ACL blocking UDP port 3784 from R2.
IS-IS BFD requires that the BFD session be established between the same IP addresses used for IS-IS. If R1's BFD session is down but R2's is up, it indicates a unidirectional issue. This can be due to an MTU mismatch where R1's BFD packets are fragmented and dropped, or a firewall blocking inbound BFD packets on R1. Here, the most likely cause is that R1 has an ACL denying UDP port 3784 from R2.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
R1 has an ACL blocking UDP port 3784 from R2.
IS-IS requires 'bfd all-interfaces' to work with BFD.
Why it's wrong here
Per-interface BFD is supported.
The BFD multiplier on R1 is too low.
Why it's wrong here
Multiplier 3 is standard.
The IS-IS metric must be set to 1 for BFD.
Why it's wrong here
Metric does not affect BFD.
Common exam traps
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
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.
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
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
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) — study guide chapter
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FAQ
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — OSPF neighbours must agree on key parameters..
The correct answer is: R1 has an ACL blocking UDP port 3784 from R2. — IS-IS BFD requires that the BFD session be established between the same IP addresses used for IS-IS. If R1's BFD session is down but R2's is up, it indicates a unidirectional issue. This can be due to an MTU mismatch where R1's BFD packets are fragmented and dropped, or a firewall blocking inbound BFD packets on R1. Here, the most likely cause is that R1 has an ACL denying UDP port 3784 from R2.
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.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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