Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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
Based on the exhibit, why is R1 not learning routes from R2 through OSPF on the shared link?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
GigabitEthernet0/0 is configured as a passive OSPF interface on R1.
This is correct because a passive OSPF interface does not send hellos and therefore will not form a neighbor adjacency on that link.
B
Distractor review
The routers must use different OSPF process IDs to exchange routes.
This is wrong because OSPF process IDs are locally significant and do not need to match.
C
Distractor review
The routers are in different IPv4 subnets.
This is wrong because both interfaces are in 10.20.12.0/24.
D
Distractor review
OSPF can advertise routes only across serial links.
This is wrong because OSPF commonly operates across Ethernet networks.
Common exam trap
Common exam trap: answer the scenario, not the keyword
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.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on the exchange of hello packets between routers on a shared network segment to establish neighbor adjacencies. These adjacencies are essential for routers to share link-state information and build a synchronized routing database. Without forming neighbor relationships, OSPF routers cannot exchange routing updates, preventing route learning. The passive-interface command in OSPF configuration disables the sending of hello packets on the specified interface. While the router continues to advertise the connected network into OSPF, it does not attempt to form neighbor adjacencies on that interface. This is useful for interfaces connected to end devices or networks where no OSPF neighbors exist. However, if mistakenly applied on an interface connecting two OSPF routers, it prevents adjacency formation and route exchange. A common exam trap involves assuming that matching OSPF process IDs or correct subnetting alone guarantees neighbor formation. In reality, the passive-interface setting overrides these factors by stopping hello packets. Practically, this means that even if network statements and IP addressing are correct, no routes will be learned if one side is passive. Understanding this behavior is critical for troubleshooting OSPF connectivity issues and ensuring proper routing in Cisco networks.
KKey Concepts to Remember
- OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
- Configuring an interface as passive in OSPF prevents the sending of hello packets, thus blocking neighbor adjacency formation on that interface.
- OSPF process IDs are locally significant and do not need to match between routers to establish neighbor relationships.
- OSPF requires interfaces to be in the same IP subnet to form neighbor adjacencies and exchange routing information.
- OSPF operates over multiple link types including Ethernet and serial, and is not limited to serial links only.
- A passive interface in OSPF still advertises its connected network but does not participate in neighbor discovery or route exchange on that link.
- Neighbor adjacency formation in OSPF depends on active hello packet exchanges, which are disabled on passive interfaces.
- Misconfiguring an interface as passive in OSPF is a common troubleshooting cause for missing routes despite correct network statements.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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