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300-410 Practice Question: Is troubleshooting an OSPF adjacency that fails…
A network engineer is troubleshooting an OSPF adjacency that fails to come up. Both routers are directly connected via a serial link. BFD is enabled on the interface. The engineer sees that the BFD session is down. The OSPF configuration shows 'ip ospf bfd' under the interface. The serial interface is up/up. What should the engineer check first?
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
✓
Verify that the 'bfd' command is configured under the OSPF process.
BFD on serial interfaces often requires the 'bfd interval' command to be explicitly configured, as the default may be disabled. Also, serial interfaces may need the 'bfd' command under the routing protocol as well.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Verify that the 'bfd' command is configured under the OSPF process.
Why this is correct
For OSPF BFD to work, the 'bfd' command must be enabled under the OSPF routing process (router ospf X, then 'bfd all-interfaces' or per-neighbor). Without it, the BFD session will not be initiated.
- ✗
Check if the serial interface is configured with 'encapsulation ppp'.
Why it's wrong here
Encapsulation type does not affect BFD; BFD works over HDLC, PPP, and Frame Relay.
- ✗
Ensure that the 'ip ospf bfd' command is also configured on the neighbor router.
Why it's wrong here
While both sides need BFD, the absence on one side would cause the BFD session to be down, but the first step is to check the routing process configuration.
- ✗
Verify that the serial interface clock rate is set correctly.
Why it's wrong here
Clock rate affects physical layer, but if the interface is up/up, clocking is likely correct.
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
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.