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300-410 Practice Question: Is troubleshooting a scenario where two routers…
A network engineer is troubleshooting a scenario where two routers running EIGRP are not forming an adjacency. Both routers have BFD configured under the EIGRP process and on the interfaces. The BFD session is up and operational. However, the EIGRP neighbor status shows 'Pending' and never transitions to 'Up'. What is the most likely 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
✓
The EIGRP K-values are mismatched between the two routers.
EIGRP requires the BFD session to be fully established before it will bring up the adjacency. If the BFD session is up but EIGRP is stuck in Pending, the issue is often a mismatch in EIGRP K-values 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.
- ✗
The BFD timers are set too high, causing EIGRP to time out before BFD can respond.
Why it's wrong here
BFD timers do not affect EIGRP's Pending state; the BFD session is already up.
- ✗
EIGRP is configured with 'no auto-summary' on one router and 'auto-summary' on the other.
Why it's wrong here
Auto-summary mismatch does not prevent adjacency formation; it affects route advertisement.
- ✓
The EIGRP K-values are mismatched between the two routers.
Why this is correct
EIGRP K-values must match for adjacency to form; a mismatch causes the neighbor to stay in Pending state even if BFD is up.
- ✗
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor expects different values.
Why it's wrong here
BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.
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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