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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
Correct because EIGRP will not form an adjacency if the interfaces are not in the same subnet, as the hello packets will be dropped.
- ✗
The EIGRP process is shut down on one of the routers.
Why it's wrong here
Incorrect because if the process is shut down, 'show ip eigrp neighbors' would still show the local process, but no neighbors. However, the more common cause is mismatched subnets.
- ✗
The passive-interface default command is configured under the EIGRP process.
Why it's wrong here
Incorrect because passive-interface default would suppress hello packets on all interfaces, but the engineer would likely see a different symptom, such as no neighbors and the interface being listed as passive.
- ✗
The EIGRP router ID is the same on both routers.
Why it's wrong here
Incorrect because a duplicate router ID does not prevent adjacency formation; it only causes issues during route exchange and may cause instability.
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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Last reviewed: Jul 4, 2026
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