CCNA IP Routing Practice Question
Exhibit
R1# show running-config | section router ospfv3 router ospfv3 1 router-id 1.1.1.1 area 0 authentication ipsec spi 256 md5 1234567890ABCDEF1234567890ABCDEF ! interface Serial0/0/0 ipv6 address 2001:DB8:1:1::1/64 ospfv3 1 ipv6 area 0 R2# show running-config | section router ospfv3 router ospfv3 1 router-id 2.2.2.2 ! interface Serial0/0/0 ipv6 address 2001:DB8:1:1::2/64 ospfv3 1 ipv6 area 0
An engineer is troubleshooting an OSPFv3 adjacency issue between two routers R1 and R2 connected over a serial link. The link is up/up on both sides, and IPv6 is enabled on the interfaces. However, the 'show ipv6 ospf neighbor' command shows no neighbors. The engineer checks the OSPFv3 configuration. What is the most likely cause of the missing adjacency?
⚠ Common exam trap
Cisco often tests the misconception that OSPFv3 requires matching subnets (like OSPFv2) or that authentication is optional, when in fact OSPFv3 uses IPsec and any mismatch breaks adjacency silently.
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
✓
OSPFv3 authentication is configured on R1 but not on R2.
OSPFv3 uses IPsec for authentication, unlike OSPFv2 which uses plaintext or MD5 authentication. If authentication is configured on one router but not the other, the OSPFv3 Hello packets will be dropped, preventing neighbor adjacency from forming. The 'show ipv6 ospf neighbor' command will show no neighbors because the routers cannot exchange Hello packets successfully.
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 serial interface on R2 is administratively down.
Why it's wrong here
If the serial interface on R2 were administratively down, the interface state would display as "administratively down" and the line protocol would be down, because the shutdown command disables the interface at Layer 1. However, the exhibit shows R2's serial interface in an up/up state, which means it is not disabled and OSPFv3 can send and receive Hellos on that link. Therefore, an administrative shutdown cannot be the reason the adjacency fails.
- ✓
OSPFv3 authentication is configured on R1 but not on R2.
Why this is correct
On R1, OSPFv3 is configured with IPsec authentication, but R2 has no matching IPsec configuration under its OSPFv3 process. OSPFv3 relies on IPsec AH or ESP for authentication and integrity, unlike OSPFv2, and if one router encrypts or authenticates its packets while the other does not, the Hellos are silently discarded and the neighbor relationship cannot progress past the EXSTART/INIT state. Both routers must use the same SPI, authentication algorithm, and key for adjacency to form.
- ✗
The IPv6 address on R2 is in a different subnet than R1.
Why it's wrong here
The IPv6 addresses on R1 and R2 both fall within the 2001:DB8:1:1::/64 prefix, so they are in the same subnet. In OSPFv3, neighbor adjacency is established using link-local addresses and does not actually require the global unicast addresses to match or even be on the same subnet. Therefore, the premise that R2's IPv6 address is in a different subnet is factually incorrect and cannot explain the problem.
- ✗
The OSPFv3 router-id is not configured on R2.
Why it's wrong here
R2 has a router-id of 2.2.2.2 explicitly configured under the ospfv3 process, so the router-id is not missing. OSPFv3 requires a 32-bit router-id, and if none is configured, the router may fail to start the OSPFv3 process or will derive one from an IPv4 address; here that is not the case. Thus, the absence of a router-id is not the cause of the adjacency failure.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓OSPFv3 authentication is configured on R1 but not on R2.Correct answer▾
Why this is correct
On R1, OSPFv3 is configured with IPsec authentication, but R2 has no matching IPsec configuration under its OSPFv3 process. OSPFv3 relies on IPsec AH or ESP for authentication and integrity, unlike OSPFv2, and if one router encrypts or authenticates its packets while the other does not, the Hellos are silently discarded and the neighbor relationship cannot progress past the EXSTART/INIT state. Both routers must use the same SPI, authentication algorithm, and key for adjacency to form.
✗The serial interface on R2 is administratively down.Wrong answer — click to see why▾
Why this is wrong here
The question states the serial link is up/up on both sides, meaning the interface is not administratively down. An administratively down interface would show as 'administratively down' in the interface status, not 'up/up'.
Why candidates choose this
Students may think that an interface being down is a common cause for OSPF adjacency failure, but the given information explicitly rules this out.
✗The IPv6 address on R2 is in a different subnet than R1.Wrong answer — click to see why▾
Why this is wrong here
Both interfaces have IPv6 addresses in the 2001:DB8:1:1::/64 subnet, so the subnet matches. OSPFv3 does not require interfaces to be in the same subnet for adjacency, but it does require link-local addresses to be reachable. However, the given addresses are global unicast, and the subnet match is not the issue here.
Why candidates choose this
In OSPFv2, mismatched subnets prevent adjacency, so students may incorrectly apply the same logic to OSPFv3. However, OSPFv3 uses link-local addresses for neighbor discovery and does not require matching global prefixes.
✗The OSPFv3 router-id is not configured on R2.Wrong answer — click to see why▾
Why this is wrong here
The router-id is configured on R2 as 2.2.2.2, as stated in the existing explanation. OSPFv3 requires a router-id, and it is present, so this is not the cause.
Why candidates choose this
A missing router-id is a common OSPF issue, but in this case it is configured. Students might overlook the configuration details and assume it is missing.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Go deeper
Related to this question
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
Key term
OSPF neighbor
An OSPF neighbor is another router that has been directly discovered through OSPF Hello packets and is willing to exchange routing information to build a network topology map.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.