- A
One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'.
Correct. Mismatched authentication modes (MD5 vs SHA-256) will prevent EIGRP neighbor formation.
- B
Both routers use the same key chain name but one router has key ID 1 with string 'cisco' and the other has key ID 2 with string 'cisco'.
Why wrong: Incorrect. Even though key IDs differ, both routers have the same key chain name and the same key string, so authentication will succeed.
- C
One router has 'ip authentication key-chain eigrp 100 MYCHAIN' configured, while the other router has no authentication configuration at all.
Why wrong: Incorrect. With no authentication on one side, the EIGRP adjacency can still form, albeit with a warning message.
- D
Both routers use the same key chain and key ID, but the key string on one router is 'cisco123' and on the other is 'Cisco123' (case-sensitive).
Correct. The key strings differ in case (case-sensitive), causing an authentication mismatch and preventing neighbor formation.
- E
The 'accept-lifetime' on one router is set to 00:00:00 Jan 1 2020 to 23:59:59 Dec 31 2020, and the current time is in 2023.
Why wrong: Incorrect. This is an expired key lifetime issue, not an authentication mismatch between two routers.
Why EIGRP Neighbor Fails Due to Authentication Mismatch
This 300-410 practice question tests your understanding of eigrp troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: eIGRP Authentication. 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.
Which TWO actions will prevent an EIGRP neighbor relationship from forming due to authentication mismatch? (Choose TWO.)
Quick Answer
The answer is that an EIGRP authentication mismatch prevents neighbor formation when both routers use the same key chain and key ID but the key strings differ in case, such as 'cisco123' versus 'Cisco123', because EIGRP authentication is case-sensitive. This occurs because MD5 or SHA-256 authentication requires the key string to be byte-for-byte identical on both sides; any difference, including capitalization, causes the hash calculation to fail, breaking the neighbor relationship. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how EIGRP authentication parameters must match exactly, and a common trap is assuming case-insensitivity or that mismatched key IDs still allow adjacency. Remember that authentication type (MD5 vs. SHA-256), key ID, key string, and time-based lifetimes must all align—a single mismatch in any of these can silently drop hello packets. Memory tip: think "CASE and KEY must AGREE" — Case-sensitive strings, Authentication type, Same key ID, and Enabled lifetimes must all be Equal for a neighbor to form.
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
One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'.
EIGRP neighbor relationships fail due to authentication mismatch when authentication modes differ (MD5 vs SHA-256) as in A, and when key strings differ in case-sensitivity as in D. Option B does NOT prevent the neighbor relationship because both routers use the same key chain and same key string; the key ID does not need to match for authentication to succeed. Option C will not prevent the relationship because if one router has no authentication, the adjacency forms with a warning. Option E describes a key expiration scenario, which prevents adjacency but is not an authentication mismatch between two routers.
Key principle: EIGRP Authentication
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'.
- ✗
Both routers use the same key chain name but one router has key ID 1 with string 'cisco' and the other has key ID 2 with string 'cisco'.
Why it's wrong here
Incorrect. Even though key IDs differ, both routers have the same key chain name and the same key string, so authentication will succeed.
- ✗
One router has 'ip authentication key-chain eigrp 100 MYCHAIN' configured, while the other router has no authentication configuration at all.
Why it's wrong here
Incorrect. With no authentication on one side, the EIGRP adjacency can still form, albeit with a warning message.
- ✓
Both routers use the same key chain and key ID, but the key string on one router is 'cisco123' and on the other is 'Cisco123' (case-sensitive).
Why this is correct
Correct. The key strings differ in case (case-sensitive), causing an authentication mismatch and preventing neighbor formation.
Related concept
EIGRP Authentication
- ✗
The 'accept-lifetime' on one router is set to 00:00:00 Jan 1 2020 to 23:59:59 Dec 31 2020, and the current time is in 2023.
Why it's wrong here
Incorrect. This is an expired key lifetime issue, not an authentication mismatch between two routers.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap is to think that mismatched authentication mode or key string prevents formation, but also consider that missing authentication on one side may not block adjacency.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- EIGRP Authentication
- Authentication Key Chain
- Accept and Send Lifetimes
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
EIGRP Authentication
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.
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.
Review eIGRP Authentication, then practise related 300-410 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — EIGRP Authentication.
What is the correct answer to this question?
The correct answer is: One router is configured with 'ip authentication mode eigrp 100 md5' and the other with 'ip authentication mode eigrp 100 sha256'. — EIGRP neighbor relationships fail due to authentication mismatch when authentication modes differ (MD5 vs SHA-256) as in A, and when key strings differ in case-sensitivity as in D. Option B does NOT prevent the neighbor relationship because both routers use the same key chain and same key string; the key ID does not need to match for authentication to succeed. Option C will not prevent the relationship because if one router has no authentication, the adjacency forms with a warning. Option E describes a key expiration scenario, which prevents adjacency but is not an authentication mismatch between two routers.
What should I do if I get this 300-410 question wrong?
Review eIGRP Authentication, then practise related 300-410 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
EIGRP Authentication
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Last reviewed: Jun 18, 2026
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