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300-410 Practice Question: An engineer is troubleshooting a routing loop…
An engineer is troubleshooting a routing loop between two routers. R1 and R2 are running both OSPF and EIGRP. R1 learns the prefix 172.16.1.0/24 via OSPF with AD 110 and via EIGRP internal with AD 90. The engineer notices that R1 installs the EIGRP route, but traffic to 172.16.1.0/24 is being dropped. What is the most likely issue?
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 route is a summary route pointing to a null0 interface.
EIGRP internal routes have AD 90, which is lower than OSPF's 110, so EIGRP is preferred. However, if the EIGRP route points to a next-hop that is not reachable or is a backup path, traffic may be dropped. The root cause could be that the EIGRP route is learned via a secondary path that is not optimal or has a next-hop issue.
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 OSPF route has a better metric, but the EIGRP route is preferred due to lower AD.
Why it's wrong here
This is expected behavior; AD is compared first, so EIGRP is preferred. The issue is not about metric.
- ✓
The EIGRP route is a summary route pointing to a null0 interface.
Why this is correct
If R1 has an EIGRP summary route for 172.16.1.0/24 pointing to Null0, it will be installed with AD 90 and drop traffic, causing a black hole.
- ✗
The OSPF route has a higher AD because it is a type 5 LSA.
Why it's wrong here
OSPF AD is 110 for all routes; type 5 does not change AD.
- ✗
The EIGRP route is an external route with AD 170.
Why it's wrong here
The question states it is an internal route with AD 90, so this is not applicable.
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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JA
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.