Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.0.12.1 255.255.255.252 ip ospf authentication message-digest ip ospf message-digest-key 1 md5 cisco123 ! router ospf 1 network 10.0.12.0 0.0.0.3 area 0 R2# interface GigabitEthernet0/0 ip address 10.0.12.2 255.255.255.252 ip ospf authentication message-digest ip ospf message-digest-key 1 md5 cisco321 ! router ospf 1 network 10.0.12.0 0.0.0.3 area 0
R1 and R2 are directly connected and running OSPF. They can ping each other, the area matches, and the timers match, but they still do not become neighbors. Based on the exhibit, what is the most likely cause?
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
The OSPF authentication keys do not match.
This is correct because mismatched OSPF MD5 authentication prevents neighbor formation even when addressing and area values are correct.
B
Distractor review
The subnet mask is too small for OSPF to operate.
This is wrong because OSPF works normally on /30 links.
C
Distractor review
The routers must use different process IDs.
This is wrong because process IDs are locally significant and do not need to match or differ.
D
Distractor review
The interfaces must be configured as switch trunks.
This is wrong because OSPF commonly runs over routed interfaces.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to overlook OSPF authentication mismatches when routers have IP connectivity and matching area IDs. Candidates may incorrectly assume that because the routers can ping each other and timers match, adjacency must form. However, if OSPF authentication keys differ, routers silently reject OSPF packets, preventing neighbor formation. This trap exploits the misconception that IP reachability alone ensures OSPF adjacency, ignoring the critical role of matching authentication parameters in the OSPF neighbor negotiation process.
Technical deep dive
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that forms neighbor adjacencies to exchange routing information. For two routers to become OSPF neighbors, they must share several key parameters: the same area ID, matching hello and dead intervals, and identical authentication settings if authentication is enabled. Authentication in OSPF can be simple password or message-digest (MD5), which secures OSPF packets by requiring matching keys on both routers. Without matching authentication keys, OSPF packets are discarded, preventing neighbor formation. The OSPF adjacency formation process involves routers exchanging hello packets to discover neighbors and negotiate parameters. If any critical parameter differs, such as area ID or authentication key, the routers will not progress beyond the init or two-way state. Process IDs are locally significant and do not need to match; they only identify the OSPF process on a router. Additionally, OSPF runs over routed interfaces and does not require switch trunking, which is relevant for VLAN tagging but unrelated to OSPF neighbor formation. A common exam trap is assuming that IP connectivity and matching area IDs guarantee OSPF adjacency. However, when authentication is enabled, mismatched keys cause silent failures where routers can ping each other but never become neighbors. This scenario highlights the importance of verifying all OSPF parameters, especially authentication keys, during troubleshooting. In practical networks, mismatched MD5 keys are a frequent cause of adjacency failures despite correct IP and area configurations.
KKey Concepts to Remember
- OSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies.
- OSPF adjacency formation requires identical area IDs, hello and dead timers, and authentication parameters on both routers.
- OSPF process IDs are locally significant and do not affect neighbor relationships between routers.
- OSPF can operate over routed point-to-point links without requiring switch trunk configurations on interfaces.
- Mismatched OSPF authentication keys cause routers to reject each other's OSPF packets despite successful IP connectivity.
- OSPF adjacency depends on protocol parameter matching beyond basic IP reachability, including authentication and area configuration.
- OSPF message-digest (MD5) authentication provides secure neighbor relationships by requiring matching keys on both ends.
- IP connectivity alone does not guarantee OSPF neighbor formation if protocol parameters such as authentication keys differ.
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 routers must have matching authentication keys and types to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: The OSPF authentication keys do not match. — The most likely cause is that the interfaces are in different OSPF areas? No — the exhibit shows the area does match. The real problem is a mismatched OSPF authentication key. In practical terms, the routers are exchanging traffic on the link and they are both trying to use OSPF with message-digest authentication, but they do not trust each other because the configured MD5 key values differ. OSPF adjacency depends on matching protocol parameters, not just IP connectivity. This is a classic troubleshooting scenario because the link can still appear healthy and correctly addressed. The failure happens at the protocol-validation stage. When authentication is enabled on both sides but the key or key ID does not match correctly, the routers reject each other as valid OSPF neighbors.
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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