Question 242 of 2,011
hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures BFD on an OSPF link that…
An engineer configures BFD on an OSPF link that is part of a virtual link. The BFD session between the two endpoints of the virtual link is up, but OSPF adjacency over the virtual link is not forming. What is the most likely explanation?
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
✓
BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces.
BFD does not support virtual links in OSPF. BFD can only be configured on physical interfaces or subinterfaces, not on virtual links. Therefore, even if BFD is up on the underlying physical link, it will not help detect failures of the virtual link. The virtual link adjacency may fail due to other issues, such as mismatched area IDs or authentication.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces.
Why this is correct
Correct. OSPF virtual links are not supported with BFD. The BFD session on the underlying physical interface does not affect the virtual link adjacency.
- ✗
The virtual link is configured with a different hello interval than the BFD interval, causing a mismatch.
Why it's wrong here
Incorrect. BFD and OSPF hello intervals are independent; they do not need to match.
- ✗
The 'bfd all-interfaces' command is not configured under the OSPF process, so OSPF ignores BFD for virtual links.
Why it's wrong here
Incorrect. Even with 'bfd all-interfaces', BFD does not apply to virtual links.
- ✗
The virtual link is using a different authentication type than the BFD session.
Why it's wrong here
Incorrect. BFD and OSPF authentication are separate; they do not need to match.
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.
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Last reviewed: Jun 18, 2026
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