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300-410 Practice Question: Is troubleshooting an EIGRP adjacency issue…
A network engineer is troubleshooting an EIGRP adjacency issue between two directly connected routers, R1 and R2. Both routers are configured with the same autonomous system number, but the adjacency fails to come up. The engineer checks the interfaces and verifies that they are up/up. On R1, the output of 'show ip eigrp neighbors' shows nothing. What is the most likely cause of this problem?
⚠ Common exam trap
Cisco often tests the requirement that EIGRP neighbors must be on the same subnet, and the trap here is that candidates may overlook this fundamental Layer 3 prerequisite and instead focus on less likely issues like process shutdown or router ID conflicts.
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 interfaces are configured with IP addresses from different subnets.
EIGRP requires that directly connected neighbors share a common subnet for their interfaces. If R1 and R2 have IP addresses from different subnets, EIGRP will not form an adjacency because the hello packets sent by one router will be considered invalid by the other due to the subnet mismatch. The 'show ip eigrp neighbors' output is empty because no neighbor has been discovered, which is consistent with this Layer 3 mismatch.
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 interfaces are configured with IP addresses from different subnets.
Why this is correct
EIGRP forms adjacencies only when the primary interface addresses share a common subnet. Mismatched subnets prevent the hello packets from being accepted as valid neighbours, so no adjacency appears despite matching autonomous system numbers and up/up interfaces.
- ✗
The EIGRP process is shut down on one of the routers.
Why it's wrong here
Shutting down the EIGRP process would remove it from the routing table entirely, yet the stem implies EIGRP is running and merely failing to form an adjacency. This is tempting because a disabled process also yields no neighbours, but it would be correct only if 'show ip protocols' showed EIGRP inactive.
- ✗
The passive-interface default command is configured under the EIGRP process.
Why it's wrong here
Passive-interface default suppresses EIGRP hellos on every interface, but the stem confirms interfaces are up/up and gives no hint of suppression; it would be correct if hellos were blocked on the transit link. The real cause is a mismatched K-value or authentication, which prevents neighbour formation.
- ✗
The EIGRP router ID is the same on both routers.
Why it's wrong here
Duplicate router IDs do not prevent EIGRP adjacency; the routers still exchange Hellos and form a neighbour relationship, with the ID only affecting route selection and loop prevention. It is tempting because duplicate IDs cause problems in OSPF, where they do block adjacency, but EIGRP treats them differently.
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.