Question 1,420 of 1,389
CCNA IP Routing Practice Question
Exhibit
R1# interface GigabitEthernet0/0 ipv6 address fe80::1 link-local ipv6 ospf 10 area 0 ! ipv6 router ospf 10 router-id 1.1.1.1 R2# interface GigabitEthernet0/0 ipv6 address fe80::2 link-local ipv6 ospf 10 area 1 ! ipv6 router ospf 10 router-id 2.2.2.2
Two directly connected routers running OSPFv3 do not form an adjacency. Both interfaces have valid IPv6 addresses and can ping each other using link-local addresses. What is the most likely cause?
⚠ Common exam trap
A frequent exam trap is to assume that because two routers can ping each other using IPv6 link-local addresses, their OSPFv3 adjacency should automatically form. This mistake overlooks the critical requirement that both routers must be configured in the same OSPFv3 area. Candidates might also incorrectly believe that global unicast addresses are necessary for OSPFv3 adjacency or that router IDs must be identical. These misconceptions lead to selecting incorrect answers, as adjacency depends on matching area IDs and unique router IDs, not on global addressing or identical IDs.
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 assigned to different OSPFv3 areas.
The most likely cause is an OSPFv3 area mismatch on the interface. In practical terms, OSPFv3 still requires neighbors on the same link to agree on the area assignment, just as OSPF for IPv4 does. Link-local reachability alone is not enough to form an adjacency. The protocol parameters still have to match. This is an important IPv6 routing point because people sometimes assume that successful IPv6 ping means the routing protocol should automatically work. It does not. Adjacency depends on protocol alignment, not just basic connectivity.
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 assigned to different OSPFv3 areas.
Why this is correct
In OSPFv3 (and OSPFv2), each interface belongs to exactly one area. Two routers directly connected on the same link must have that link in the same area to become neighbors; if they differ, the Hello packets are rejected because the Area ID is carried in the OSPF header, and the neighbor relationship stays down.
- ✗
The routers need global unicast addresses before OSPFv3 can run.
Why it's wrong here
OSPFv3's transport is based on IPv6 link-local addresses; it does not require global unicast addresses to establish the neighbor state. The OSPFv3 Hello protocol uses link-local destinations (FF02::5) and source addresses, so the adjacency can form even if global addresses are absent or not yet assigned. Global addresses are only necessary for external route advertisement or certain network designs, not for basic neighbor formation.
When this WOULD be correct
In a different scenario where the question specifies that OSPFv3 is configured to require global unicast addresses for its operation, and the routers are only configured with link-local addresses, this option would be correct as it would prevent adjacency formation.
- ✗
The router IDs must be identical before adjacency can form.
Why it's wrong here
OSPFv3 requires each router to have a unique 32-bit Router ID, typically derived from an IPv4 address, to identify itself in the routing domain. If two routers had identical Router IDs, the adjacency would not form correctly; instead, it would cause a conflict and one of them would be rejected. The actual requirement is that Router IDs are unique, not identical, for a neighbor relationship to succeed.
When this WOULD be correct
In a different scenario, if the question specified that both routers were configured with the same router ID, and that was a requirement for forming an adjacency, then this option would be correct. For example, if the question stated that the routers were misconfigured to have the same router ID, it would prevent adjacency.
- ✗
The link-local addresses must be learned from DHCPv6.
Why it's wrong here
IPv6 link-local addresses (FE80::/10) are automatically configured on every interface using EUI-64 or random generation, independent of DHCPv6. OSPFv3 uses these link-local addresses as the source for Hello packets and routing updates, so no DHCPv6 involvement is needed for adjacency formation. Even if DHCPv6 were used for other address assignment, it is irrelevant to the OSPFv3 neighbor process.
When this WOULD be correct
In a different scenario where the question specifies that OSPFv3 is configured on routers that only have link-local addresses and no global unicast addresses are assigned, this option would be correct. The question could state that the routers are unable to form an OSPFv3 adjacency due to the absence of global unicast addresses, which are necessary for routing beyond the local link.
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.
✓The interfaces are assigned to different OSPFv3 areas.Correct answer▾
Why this is correct
In OSPFv3 (and OSPFv2), each interface belongs to exactly one area. Two routers directly connected on the same link must have that link in the same area to become neighbors; if they differ, the Hello packets are rejected because the Area ID is carried in the OSPF header, and the neighbor relationship stays down.
✗The routers need global unicast addresses before OSPFv3 can run.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPFv3 can operate using link-local addresses, and global unicast addresses are not a requirement for forming OSPFv3 adjacencies. The routers can still communicate using their link-local addresses.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPFv3 is configured to require global unicast addresses for its operation, and the routers are only configured with link-local addresses, this option would be correct as it would prevent adjacency formation.
Why candidates choose this
Candidates may choose this option due to a common misconception that OSPF requires global unicast addresses, leading them to overlook the capabilities of OSPFv3 with link-local addresses.
✗The router IDs must be identical before adjacency can form.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPFv3 does not require router IDs to be identical for adjacency to form; each router can have a unique router ID. Adjacency formation depends on other factors such as matching OSPF parameters and area configurations.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that both routers were configured with the same router ID, and that was a requirement for forming an adjacency, then this option would be correct. For example, if the question stated that the routers were misconfigured to have the same router ID, it would prevent adjacency.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of OSPFv3 requirements, conflating the need for unique router IDs with adjacency formation, as they may have encountered similar concepts in other routing protocols.
✗The link-local addresses must be learned from DHCPv6.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPFv3 can operate using link-local addresses without the need for global unicast addresses. The OSPFv3 protocol is designed to work with link-local addresses for neighbor discovery and adjacency formation.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPFv3 is configured on routers that only have link-local addresses and no global unicast addresses are assigned, this option would be correct. The question could state that the routers are unable to form an OSPFv3 adjacency due to the absence of global unicast addresses, which are necessary for routing beyond the local link.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of OSPFv3 requirements, mistakenly believing that global unicast addresses are mandatory for any OSPFv3 operation, leading to confusion about the protocol's capabilities.
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.
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Last reviewed: May 17, 2026
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.
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