Question 1,838 of 2,152

EIGRP Troubleshooting →mediumMultiple ChoiceObjective-mapped

EIGRP Neighbor Not Forming Due to Subnet Mismatch

This 300-410 practice question tests your understanding of eigrp troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

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?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The interfaces are configured with IP addresses from different subnets.
Correct because EIGRP will not form an adjacency if the interfaces are not in the same subnet, as the hello packets will be dropped.
B
The EIGRP process is shut down on one of the routers.
Why wrong: 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.
C
The passive-interface default command is configured under the EIGRP process.
Why wrong: 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.
D
The EIGRP router ID is the same on both routers.
Why wrong: Incorrect because a duplicate router ID does not prevent adjacency formation; it only causes issues during route exchange and may cause instability.

Full breakdown with real-world context →

Quick Answer

The answer is a subnet mismatch between the directly connected interfaces. EIGRP requires that the primary IP addresses on the same link belong to the exact same subnet; if the subnet masks differ, each router considers the other to be on a different network, and the hello packets are silently discarded, preventing adjacency formation. On the Cisco CCNP ENARSI 300-410 exam, this is a classic troubleshooting scenario where you must verify that both interface IPs share an identical prefix length, as a common trap is assuming a /24 mask on one side and a /30 on the other will still work. A quick memory tip: “Same subnet, same mask—EIGRP won’t ask.”

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.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.

Common exam traps

Common exam trap: answer the scenario, not the keyword

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.

Trap categories for this question

Command / output trap
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.

Detailed technical explanation

How to think about this question

EIGRP uses multicast hello packets (224.0.0.10) to discover neighbors, and these packets are only accepted if the source IP address belongs to the same subnet as the receiving interface. The 'show ip eigrp neighbors' command displays only routers that have exchanged valid hellos; a subnet mismatch causes the receiving router to silently discard the hello, so no neighbor entry is created. In real-world scenarios, misconfigured IP addressing is a common initial misstep during router deployment or when connecting different network segments.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

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.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: 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.

What should I do if I get this 300-410 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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