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 MTU 1500 bytes R2# show ip ospf interface g0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.12.2/30, Area 0 MTU 1400 bytes
R1 and R2 are directly connected and running OSPF. The IP addressing is correct and both routers are in area 0, but they do not form an adjacency. What is the most likely cause?
⚠ Common exam trap
Don't overlook MTU settings when OSPF adjacency issues arise, especially when basic connectivity is confirmed.
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 OSPF MTU values do not match on the connected interfaces.
The most likely cause is an MTU mismatch between the two interfaces. During the OSPF database exchange process, neighbors must agree on the MTU value to successfully exchange DBD packets; a mismatch typically causes the adjacency to become stuck in the ExStart or Exchange state. Basic connectivity (e.g., ping) often still works because ICMP packets are small, but the OSPF adjacency fails due to the MTU discrepancy.
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 MTU values do not match on the connected interfaces.
Why this is correct
When OSPF neighbors exchange Database Description (DD) packets, each packet includes an interface MTU field. If the MTU values differ, the receiving router detects a mismatch and drops the DD packet, leaving the adjacency stuck in the ExStart or Exchange state. This prevents the routers from completing the database synchronization process, so the OSPF neighbor relationship cannot come up.
- ✗
The routers must use identical hostnames before OSPF can form neighbors.
Why it's wrong here
OSPF neighbor formation is determined entirely by protocol parameters such as Router ID, area ID, timers, authentication, and network type. Hostnames are only a locally configured label used for display purposes and are never transmitted in OSPF packets. Therefore, having different hostnames has no effect on OSPF's ability to establish adjacency.
When this WOULD be correct
In a different scenario where the question specifies that OSPF adjacency formation is contingent upon matching router configurations, including hostnames for identification purposes in a lab setup, this option could be correct.
- ✗
The routers must use VLAN 1 for OSPF to operate.
Why it's wrong here
OSPF operates at Layer 3 and builds adjacencies over IP interfaces, completely independent of any Layer 2 VLAN configuration. The use of VLAN 1 is a switch-specific administrative default, but routed interfaces can belong to any VLAN or no VLAN at all; OSPF will function identically as long as IP reachability exists between the routers.
When this WOULD be correct
In a different scenario where the question specifies that OSPF is only configured to operate on VLAN 1, and both routers are incorrectly configured on different VLANs, this option would be correct. The question would need to emphasize that OSPF requires VLAN 1 for adjacency.
- ✗
The interfaces need to be configured as trunks instead of routed ports.
Why it's wrong here
Trunks are Layer 2 constructs used to carry multiple VLANs over a single link and are irrelevant to OSPF, a Layer 3 protocol. For OSPF to form an adjacency, the interfaces must have IP addresses assigned and be configured as routed ports, not switchports. Using trunks would actually prevent OSPF from running directly on those interfaces unless subinterfaces are used.
When this WOULD be correct
In a different scenario where the question specifies that OSPF is being used in a network with VLANs and requires trunking for inter-VLAN routing, this option could be correct. For example, if the question states that OSPF must be configured on trunk interfaces to communicate between different VLANs, then this option would apply.
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 OSPF MTU values do not match on the connected interfaces.Correct answer▾
Why this is correct
When OSPF neighbors exchange Database Description (DD) packets, each packet includes an interface MTU field. If the MTU values differ, the receiving router detects a mismatch and drops the DD packet, leaving the adjacency stuck in the ExStart or Exchange state. This prevents the routers from completing the database synchronization process, so the OSPF neighbor relationship cannot come up.
✗The routers must use identical hostnames before OSPF can form neighbors.Wrong answer — click to see why▾
Why this is wrong here
OSPF neighbor formation does not depend on hostnames; it relies on matching parameters such as area ID, authentication, hello/dead intervals, and network type. Hostnames are only used for local identification and do not affect OSPF adjacency.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF adjacency formation is contingent upon matching router configurations, including hostnames for identification purposes in a lab setup, this option could be correct.
Why candidates choose this
Students might confuse the requirement for matching OSPF router IDs (which are typically IP addresses) with hostnames, or they may think that consistent naming is necessary for network protocols to communicate.
✗The routers must use VLAN 1 for OSPF to operate.Wrong answer — click to see why▾
Why this is wrong here
OSPF can operate over any VLAN or routed interface; there is no requirement to use VLAN 1. The adjacency issue is unrelated to VLAN numbering, and OSPF works independently of VLAN assignments on routed ports.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF is only configured to operate on VLAN 1, and both routers are incorrectly configured on different VLANs, this option would be correct. The question would need to emphasize that OSPF requires VLAN 1 for adjacency.
Why candidates choose this
Some might think that VLAN 1 is the default VLAN and therefore necessary for routing protocols, but this is incorrect because OSPF runs at Layer 3 and does not depend on specific VLANs.
✗The interfaces need to be configured as trunks instead of routed ports.Wrong answer — click to see why▾
Why this is wrong here
OSPF is designed to run on routed interfaces (Layer 3 interfaces) and does not require trunking. Trunk ports are used for carrying multiple VLANs between switches, not for OSPF adjacency between routers.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF is being used in a network with VLANs and requires trunking for inter-VLAN routing, this option could be correct. For example, if the question states that OSPF must be configured on trunk interfaces to communicate between different VLANs, then this option would apply.
Why candidates choose this
Test-takers may confuse the need for trunking in switch-to-switch connections with router-to-router connections, or they may think that OSPF requires a specific encapsulation like dot1q, which is not true for point-to-point links.
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
MTU
MTU (Maximum Transmission Unit) is the largest size of a single data packet that can be sent over a network without needing to be fragmented.
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.
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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.