OSPF Adjacency Troubleshooting: Point-to-Point Link Up but No Neighbor
You are a network engineer for a large enterprise deploying a new data center using a spine-and-leaf architecture with Juniper QFX5100 switches. The underlay network uses OSPF for loopback reachability, and the overlay uses EBGP for EVPN. The leaf switches are configured as VTEPs (Virtual Tunnel Endpoints). One of the leaf switches, leaf-03, cannot establish OSPF adjacency with its spine switch, spine-01. The interfaces are up/up and the IP addresses are correctly configured. 'show ospf neighbor' on leaf-03 returns nothing. 'show ospf interface' shows the interface is in state DOWN. Both switches are configured with the same OSPF area (0.0.0.0) and the same hello interval (10 seconds). The MTU on both sides is 1500. Authentication is not configured. The spine switch has multiple OSPF neighbors from other leaves. The network is in production and other leaf switches are working fine. What is the most likely cause of the problem?
Quick Answer
The answer is that OSPF is disabled on the interface at the leaf switch. This is the most likely cause because when a point-to-point link is up at Layer 1 but the OSPF interface state shows DOWN in the output of show ospf interface, it indicates that OSPF is not actively running on that specific interface, often due to an explicit disable command or a passive interface configuration that prevents OSPF from sending hellos. On the JNCIA-Junos exam, this scenario tests your understanding that OSPF adjacency troubleshooting must distinguish between Layer 1/2 connectivity and the OSPF process itself—a common trap is to assume an MTU or network type mismatch, but those would leave the interface state UP while preventing adjacency formation. Remember that if the interface is up/up but OSPF shows DOWN, the protocol is not enabled there; a quick memory tip is “Layer 1 up, OSPF down? Check the protocol config, not the cable.”
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
✓
OSPF is disabled on the interface at the leaf switch.
The OSPF interface is in state DOWN while the physical interface is up/up, indicating that OSPF is not enabled on that interface. This can happen if OSPF is explicitly disabled under protocol configuration or if the interface is not included in the OSPF configuration. Option A is wrong because network type mismatch would not cause the interface state to be DOWN; adjacency would fail but the interface would remain UP. Option B is wrong because MTU issues would cause OSPF adjacency problems but the interface state would be UP (not DOWN). Option D is wrong because duplicate router IDs would affect all adjacencies, not just this one, and other leaves are working fine.
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 OSPF network type is not consistent between the two switches.
Why it's wrong here
Network type mismatch would show interface state as up but adjacency fails.
- ✗
The interface MTU is misconfigured on one side.
Why it's wrong here
MTU mismatch would not cause OSPF interface to be down; adjacency would fail but interface state remains up.
- ✓
OSPF is disabled on the interface at the leaf switch.
Why this is correct
If OSPF is not enabled on the interface, the OSPF interface state will be down.
- ✗
Duplicate router ID on leaf-03 and spine-01.
Why it's wrong here
A duplicate router ID would cause problems, but other leaves work with spine-01.
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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About these practice questions
This JN0-106 question is part of Courseiva's 156-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way this is tested on JN0-106
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Two routers are connected via a point-to-point Ethernet link. They are configured with IP addresses in the same subnet, but OSPF does not form an adjacency. The link is up/up. What is a likely cause?
hard- A.The routers are in different OSPF areas
- B.OSPF authentication is configured on one router but not the other
- C.The routers have the same router ID
- ✓ D.The OSPF hello and dead intervals are mismatched
Why D: OSPF requires that Hello and Dead intervals match between neighbors on a point-to-point link to form an adjacency. Even though the link is up/up and IP addresses are in the same subnet, mismatched timers prevent the routers from agreeing on neighbor state, so no adjacency forms.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 practice question is part of Courseiva's free Juniper Networks 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 JN0-106 exam.