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300-410 Practice Question: R1 and R2 are OSPF neighbors over a VLAN…
R1 and R2 are OSPF neighbors over a VLAN interface with BFD enabled. R1#show ip ospf interface vlan10 shows 'BFD is enabled' but R1#show bfd neighbors shows no sessions. R2#show bfd neighbors shows no sessions. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under vlan10. R2 has same. The VLAN is up. What is the root cause?
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 must be enabled on the physical interface as well as the VLAN interface.
BFD on VLAN interfaces requires that the underlying physical interface also support BFD. If the physical interface (e.g., GigabitEthernet0/1) does not have BFD enabled, the VLAN interface cannot establish BFD sessions. BFD is not automatically inherited from VLAN to physical.
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 must be enabled on the physical interface as well as the VLAN interface.
Why this is correct
BFD sessions are established on the physical interface; without BFD on the physical, the VLAN interface BFD configuration is ineffective.
- ✗
The VLAN interface must have 'ip ospf bfd' disabled.
Why it's wrong here
Disabling would not help.
- ✗
OSPF must be configured with 'bfd all-interfaces' globally.
Why it's wrong here
Per-interface BFD is sufficient.
- ✗
The BFD timers must match exactly on both sides.
Why it's wrong here
Timers are negotiated; mismatch does not prevent session establishment.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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