CCNA IP Routing Practice Question
Exhibit
R1# show ip ospf interface g0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.12.1/30, Area 0 Network Type POINT_TO_POINT R2# show ip ospf interface g0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.12.2/30, Area 0 Network Type BROADCAST
A router pair is directly connected, but they do not become OSPF neighbors. IP addressing and area assignment are correct. What is the most likely cause?
⚠ Common exam trap
A common exam trap is assuming that OSPF process IDs must match on both routers to form neighbors. Many candidates mistakenly believe process IDs are globally significant, but they are only locally important identifiers. Another tempting mistake is blaming IP addressing or area mismatches without checking the OSPF network type. Since network type controls how OSPF hellos are sent and received, a mismatch between broadcast and point-to-point types can silently block adjacency formation even when IP and area configurations appear correct. This subtlety often leads to confusion during troubleshooting and exam scenarios.
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 network type mismatch on the connected interfaces
OSPF network type mismatch is a frequent reason for adjacency failure when basic IP and area settings are correct. If one side is configured as broadcast and the other as point-to-point, the hello timers, neighbor discovery behavior, and designated router election rules diverge, preventing neighbor formation. Unlike process IDs (which are locally significant), a mismatch in network type directly affects how OSPF hellos are processed. This is a well-known L2/L3 misconfiguration that must be checked alongside router‑ID and authentication parameters.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
OSPF network type mismatch on the connected interfaces
Why this is correct
One side configured as point-to-point expects no DR/BDR election and often uses a different hello/dead interval, while the other side as broadcast conducts DR/BDR elections and relies on matching timers. This mismatch prevents the routers from reaching the two-way state, so they never form a stable adjacency even though the link is physically up.
- ✗
Duplicate default routes on both routers
Why it's wrong here
Duplicate default routes are a routing-table or path-selection issue, not an OSPF adjacency issue. OSPF neighbor formation depends on successful hello exchange and matching interface parameters, not on what prefixes are in the RIB. Two default routes might cause instability or suboptimal forwarding, but they would not stop the routers from exchanging hello packets or becoming neighbors.
When this WOULD be correct
In a different scenario where the question involves troubleshooting routing issues in a network with multiple routing protocols, and the presence of duplicate default routes is explicitly stated as causing routing conflicts, this option would be correct.
- ✗
Missing VLAN trunking on the link
Why it's wrong here
VLAN trunking is a Layer 2 switch feature and is irrelevant on a directly connected routed OSPF link. The interfaces in question are OSPF-enabled routed ports, not switchports, so there is no VLAN tag negotiation needed between them. Even if a trunk were present on a switch path, a missing allowed VLAN would block connectivity, but here the scenario specifies a direct router pair with routed interfaces.
When this WOULD be correct
In a scenario where the question specifies that OSPF is being used over a trunk link between two switches, and the interfaces are configured as access ports instead of trunk ports, this option would be correct. The lack of trunking would prevent OSPF from seeing the necessary subnets to form neighbors.
- ✗
The OSPF process IDs are required to match
Why it's wrong here
OSPF process IDs are locally significant identifiers used only on the router itself; they do not affect neighbor negotiation. Two routers can use different process IDs and still become OSPF neighbors as long as area IDs, network types, timers, and authentication match. Requiring process IDs to match is a common misconception, but the OSPF protocol exchanges hello packets that carry router IDs, not process IDs.
When this WOULD be correct
In a different scenario where the question specifies that two routers are configured with different OSPF process IDs and are unable to establish adjacency, this option would be correct. For example, if the question indicates that both routers are configured with different OSPF process IDs but are intended to be in the same OSPF area, then this would be the cause of the issue.
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.
✓OSPF network type mismatch on the connected interfacesCorrect answer▾
Why this is correct
One side configured as point-to-point expects no DR/BDR election and often uses a different hello/dead interval, while the other side as broadcast conducts DR/BDR elections and relies on matching timers. This mismatch prevents the routers from reaching the two-way state, so they never form a stable adjacency even though the link is physically up.
✗Duplicate default routes on both routersWrong answer — click to see why▾
Why this is wrong here
Duplicate default routes do not affect OSPF neighbor formation; they only impact routing table entries and potential routing loops. The issue here is about the adjacency process, which relies on Hello packets and matching parameters, not on route advertisements.
★ When this WOULD be the correct answer
In a different scenario where the question involves troubleshooting routing issues in a network with multiple routing protocols, and the presence of duplicate default routes is explicitly stated as causing routing conflicts, this option would be correct.
Why candidates choose this
Students might confuse routing issues with neighbor formation, thinking that duplicate routes could cause instability that prevents adjacency. However, OSPF neighbor formation is independent of the routes being advertised.
✗Missing VLAN trunking on the linkWrong answer — click to see why▾
Why this is wrong here
VLAN trunking is a Layer 2 concept used on switch-to-switch or switch-to-router links to carry multiple VLANs. The scenario describes a router pair directly connected via routed interfaces, which operate at Layer 3 and do not require trunking.
★ When this WOULD be the correct answer
In a scenario where the question specifies that OSPF is being used over a trunk link between two switches, and the interfaces are configured as access ports instead of trunk ports, this option would be correct. The lack of trunking would prevent OSPF from seeing the necessary subnets to form neighbors.
Why candidates choose this
Test-takers might assume that any link between routers needs trunking if they are familiar with switch configurations, but OSPF neighbor formation over routed interfaces does not involve VLANs.
✗The OSPF process IDs are required to matchWrong answer — click to see why▾
Why this is wrong here
OSPF process IDs are locally significant and do not need to match between routers. Neighbor formation only requires matching area IDs, network types, Hello/Dead intervals, and authentication if configured.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that two routers are configured with different OSPF process IDs and are unable to establish adjacency, this option would be correct. For example, if the question indicates that both routers are configured with different OSPF process IDs but are intended to be in the same OSPF area, then this would be the cause of the issue.
Why candidates choose this
Many students mistakenly believe that OSPF process IDs must match because they are used to configure OSPF, but they are only used to identify the OSPF process on the local router.
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
OSPF
OSPF is a link-state routing protocol that uses the SPF algorithm to compute the shortest path to each destination within a single autonomous system.
Key term
Designated Router
A router elected in an OSPF multi-access network to manage link-state updates and reduce routing protocol traffic.
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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.