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300-410 Practice Question: In a VRF-Lite environment, EIGRP is configured…
In a VRF-Lite environment, EIGRP is configured between two routers. The engineer notices that the EIGRP neighbor relationship is flapping intermittently. Debug output shows 'dually' messages and the route is occasionally marked as 'stuck-in-active' (SIA). The link is Ethernet with no errors. Which 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
✓
A unidirectional link issue is present, where EIGRP packets are successfully sent but not received, causing the query process to time out.
EIGRP uses the Reliable Transport Protocol (RTP) for updates, queries, and replies. If there is a unidirectional link issue (e.g., one direction has high latency or packet loss), the query process may not receive replies in time, causing the route to become SIA. This is a classic edge case where the link appears operational but is unidirectional for EIGRP packets.
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 EIGRP K-values are mismatched between the two routers, causing the neighbor to reset.
Why it's wrong here
K-value mismatch prevents the neighbor from forming at all, not flapping with SIA.
- ✓
A unidirectional link issue is present, where EIGRP packets are successfully sent but not received, causing the query process to time out.
Why this is correct
Unidirectional link causes queries to be sent but replies not received, leading to SIA and neighbor flapping.
- ✗
The EIGRP stub routing feature is enabled on one router, preventing query propagation and causing the active process to hang.
Why it's wrong here
Stub routing prevents queries from being sent to the stub router, but it does not cause SIA; it actually reduces the chance of SIA.
- ✗
The 'eigrp log-neighbor-changes' command is causing excessive logging, which delays EIGRP processing.
Why it's wrong here
Logging does not cause SIA or flapping; it only records events.
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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