The correct answer is that BGP will use BFD for fast failure detection, and if BFD goes down, BGP will reset the session immediately. This is because the `neighbor fall-over bfd` command decouples BGP’s failure detection from its own hold timer, relying instead on Bidirectional Forwarding Detection to provide sub-second link-state changes. BFD’s rapid hello and dead-interval mechanism (configured here with 200 ms intervals and a multiplier of 4) detects failures far faster than BGP’s default 180-second hold timer, so when BFD signals a loss of connectivity, BGP tears down the peering session without waiting for its own timer to expire. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of how BGP integrates with BFD for convergence optimization—a common trap is assuming BFD replaces BGP keepalives entirely, but it only accelerates failure notification. Memory tip: think “BFD = BGP’s fast-forward button for failure detection.”
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Consider the following BGP configuration with BFD:
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
BGP will use BFD for fast failure detection; if BFD goes down, BGP will reset the session immediately.
The 'neighbor fall-over bfd' command enables BGP to use BFD for fast failure detection. When BFD detects a failure, BGP will immediately tear down the peering session without waiting for the hold timer.
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.
✗
BGP will use BFD to detect link failures, but the BGP hold timer still applies.
Why it's wrong here
When BFD is used, BGP bypasses the hold timer and reacts immediately to BFD down events.
✗
BGP will ignore the BFD session and continue using its own keepalive/hold mechanism.
Why it's wrong here
The 'fall-over bfd' command specifically tells BGP to use BFD.
✓
BGP will use BFD for fast failure detection; if BFD goes down, BGP will reset the session immediately.
Why this is correct
Correct. BGP registers with BFD and upon BFD session failure, BGP tears down the neighbor without waiting for the hold timer.
The 'fall-over bfd' command is only needed if BFD timers are less than 100 ms.
Why it's wrong here
The command is independent of timer values; it enables BFD integration.
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 'fall-over bfd' command specifically tells BGP to use BFD.
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: BGP will use BFD for fast failure detection; if BFD goes down, BGP will reset the session immediately. — The 'neighbor fall-over bfd' command enables BGP to use BFD for fast failure detection. When BFD detects a failure, BGP will immediately tear down the peering session without waiting for the hold timer.
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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