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300-410 Practice Question: R1 and R2 are OSPF neighbors over a tunnel…
R1 and R2 are OSPF neighbors over a tunnel interface with BFD enabled. R1#show ip ospf interface tunnel0 shows 'BFD is enabled' but R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Down'. The tunnel is up and OSPF adjacency is full. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under tunnel0. R2 has same. 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 is not supported over tunnel interfaces; it requires physical interfaces.
BFD over tunnel interfaces requires that the underlying transport (e.g., GRE) supports BFD. BFD cannot be established over a tunnel if the tunnel endpoint IP addresses are not directly connected; BFD requires physical connectivity. Since OSPF is full, the tunnel is working, but BFD fails because the tunnel is considered a virtual interface and BFD expects a direct physical link.
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 is not supported over tunnel interfaces; it requires physical interfaces.
Why this is correct
BFD cannot be used over tunnel interfaces because BFD requires direct layer 2 connectivity; tunnels are layer 3 virtual interfaces.
- ✗
The tunnel mode must be changed to 'ipsec' for BFD to work.
Why it's wrong here
IPsec does not enable BFD over tunnels.
- ✗
OSPF must be configured with 'bfd all-interfaces' to work over tunnels.
Why it's wrong here
BFD over tunnels is not supported regardless of global configuration.
- ✗
The BFD timers must be increased for tunnel interfaces.
Why it's wrong here
Timers do not enable BFD over unsupported interfaces.
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.