CCNA IP Routing Practice Question
Exhibit
R1# router ospf 1 network 10.20.12.0 0.0.0.255 area 0 passive-interface GigabitEthernet0/0 ! interface GigabitEthernet0/0 ip address 10.20.12.1 255.255.255.0 R2# router ospf 1 network 10.20.12.0 0.0.0.255 area 0 ! interface GigabitEthernet0/0 ip address 10.20.12.2 255.255.255.0
R1 and R2 are connected via a GigabitEthernet link in the same IPv4 subnet, and both routers have OSPF configured in the same area. However, R1 is not learning any OSPF routes from R2. What is the most likely cause?
⚠ Common exam trap
A frequent exam trap is to incorrectly believe that OSPF process IDs must match between routers to exchange routes or that OSPF only works on serial links. Candidates may also overlook the passive-interface setting, assuming that the presence of correct network statements guarantees neighbor formation. The passive-interface command disables hello packets, which are essential for OSPF adjacency. This subtle configuration detail often causes confusion because the router still advertises the network but refuses to form neighbors, leading to missing routes despite seemingly correct OSPF setup.
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
✓
GigabitEthernet0/0 is configured as a passive OSPF interface on R1.
The most likely reason is that one side has the interface configured as passive, which prevents OSPF hello packets from being sent on that interface. In practical terms, the network statement alone does not guarantee neighbor formation. OSPF still needs active neighbor discovery on the link. If the interface is passive, the router advertises the connected network into OSPF but does not attempt to form an adjacency there. This is a realistic routing troubleshooting pattern because the configuration can look mostly correct until you inspect the passive-interface setting.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
GigabitEthernet0/0 is configured as a passive OSPF interface on R1.
Why this is correct
GigabitEthernet0/0 being configured as a passive OSPF interface on R1 suppresses all OSPF hello packets on that link. Because R1 never sends hellos, it cannot establish a two-way neighbor relationship with R2; OSPF adjacency cannot form, and no OSPF routes are exchanged. This explains why R1 has no routes from R2 even though the physical link is operational.
- ✗
The routers must use different OSPF process IDs to exchange routes.
Why it's wrong here
OSPF process IDs are locally significant identifiers that only matter on the router where they are configured; they do not need to match between neighboring routers for OSPF to work. R1 and R2 could run process IDs 1 and 100 respectively and still exchange routes if they have compatible Area IDs, timers, and authentication settings. Thus, differing process IDs cannot be the reason for R1 not learning routes.
When this WOULD be correct
In a different scenario where the question specifies that R1 and R2 are configured with different OSPF process IDs, this option would be correct. For example, if the question asked why R1 cannot learn routes from R2 when both routers are configured with distinct OSPF process IDs, this option would accurately explain the situation.
- ✗
The routers are in different IPv4 subnets.
Why it's wrong here
Both routers are configured with addresses in the 10.20.12.0/24 subnet, so they are on the same IPv4 subnet; for example, R1 may have 10.20.12.1 and R2 10.20.12.2. OSPF on a broadcast network requires neighbors to share the same subnet to form an adjacency, and that condition is satisfied here. A subnet mismatch would show many OSPF neighbor state messages about invalid or mismatched masks, but it is not the cause of R1 missing routes.
When this WOULD be correct
In a different scenario, if the question stated that R1 and R2 are configured in separate OSPF areas and are indeed in different IPv4 subnets, this option would be correct. The question would need to focus on the inability of OSPF to exchange routes across different subnets without proper routing protocols or configurations.
- ✗
OSPF can advertise routes only across serial links.
Why it's wrong here
OSPF is not limited to serial links; it runs over any IP-capable interface, including GigabitEthernet, FastEthernet, and VLAN interfaces. On Ethernet interfaces, OSPF uses multicast address 224.0.0.5 for hello packets and automatically adapts to the broadcast network type. Therefore, the statement that OSPF can advertise only across serial links is false and irrelevant to the problem.
When this WOULD be correct
In a different exam scenario where the question specifies that OSPF is only configured on serial interfaces and the routers are connected via Ethernet, this option could be correct. The question would need to clarify that OSPF is restricted to serial links due to specific network design constraints.
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.
✓GigabitEthernet0/0 is configured as a passive OSPF interface on R1.Correct answer▾
Why this is correct
GigabitEthernet0/0 being configured as a passive OSPF interface on R1 suppresses all OSPF hello packets on that link. Because R1 never sends hellos, it cannot establish a two-way neighbor relationship with R2; OSPF adjacency cannot form, and no OSPF routes are exchanged. This explains why R1 has no routes from R2 even though the physical link is operational.
✗The routers must use different OSPF process IDs to exchange routes.Wrong answer — click to see why▾
Why this is wrong here
OSPF process IDs are locally significant and do not need to match between routers for neighbor adjacency to form. The process ID only identifies the OSPF process on the local router, so different process IDs do not prevent route exchange.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that R1 and R2 are configured with different OSPF process IDs, this option would be correct. For example, if the question asked why R1 cannot learn routes from R2 when both routers are configured with distinct OSPF process IDs, this option would accurately explain the situation.
Why candidates choose this
Students often confuse OSPF process IDs with other routing protocols like EIGRP, where the autonomous system number must match. They may incorrectly assume that OSPF process IDs must be consistent across routers.
✗The routers are in different IPv4 subnets.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both interfaces are in the same subnet (10.20.12.0/24), so subnet mismatch is not the issue. OSPF requires interfaces to be in the same subnet to form a neighbor adjacency.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that R1 and R2 are configured in separate OSPF areas and are indeed in different IPv4 subnets, this option would be correct. The question would need to focus on the inability of OSPF to exchange routes across different subnets without proper routing protocols or configurations.
Why candidates choose this
Subnet mismatch is a common OSPF neighbor issue, and test-takers may jump to this conclusion without verifying the actual IP addresses and subnet masks in the exhibit.
✗OSPF can advertise routes only across serial links.Wrong answer — click to see why▾
Why this is wrong here
OSPF can operate over any type of IP network, including Ethernet, Frame Relay, and serial links. The statement that OSPF can only advertise routes across serial links is factually incorrect.
★ When this WOULD be the correct answer
In a different exam scenario where the question specifies that OSPF is only configured on serial interfaces and the routers are connected via Ethernet, this option could be correct. The question would need to clarify that OSPF is restricted to serial links due to specific network design constraints.
Why candidates choose this
Some older OSPF implementations or exam scenarios emphasize serial links, leading students to believe OSPF is limited to serial connections. However, OSPF is widely used on Ethernet networks.
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
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
About these practice questions
One of 1,389 original 200-301 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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.