Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.1.12.1 255.255.255.0 ip ospf authentication message-digest ip ospf message-digest-key 1 md5 cisco123 ! router ospf 10 network 10.1.12.0 0.0.0.255 area 0 R2# interface GigabitEthernet0/0 ip address 10.1.12.2 255.255.255.0 ! router ospf 10 network 10.1.12.0 0.0.0.255 area 0
R1 and R2 are directly connected and running OSPF. They can ping each other, and the area assignment matches, 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
Distractor review
The routers are in different OSPF areas.
This is wrong because both routers place the subnet into area 0.
B
Best answer
The OSPF authentication settings do not match.
This is correct because one side expects MD5 authentication and the other side is not shown with matching authentication.
C
Distractor review
The routers need identical hostnames before adjacency can form.
This is wrong because hostnames do not determine OSPF adjacency.
D
Distractor review
The subnet mask prevents OSPF multicast traffic.
This is wrong because the /24 mask does not block OSPF multicast here.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that successful ping and matching OSPF areas guarantee neighbor adjacency. Candidates often overlook OSPF authentication mismatches, especially when one router uses message-digest authentication and the other does not. This leads to confusion because the link appears operational at Layer 3, but OSPF packets are discarded silently. The trap exploits the misconception that IP connectivity alone is sufficient for OSPF adjacency, ignoring the protocol’s security requirements.
Technical deep dive
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that requires routers to establish neighbor adjacencies before exchanging routing information. This process involves routers sending hello packets to discover and maintain neighbor relationships. For adjacency to form, routers must agree on several parameters including the OSPF area, hello and dead intervals, subnet mask, and authentication settings. Authentication ensures that only trusted routers participate in the OSPF domain, enhancing network security. When OSPF authentication is enabled, routers use either simple password or message-digest (MD5) authentication to verify the identity of neighbors. Both routers on a link must have matching authentication types and keys configured; otherwise, they will reject each other's OSPF packets. Even if routers can ping each other and share the same area, a mismatch in authentication settings prevents neighbor adjacency formation because the OSPF protocol treats unauthenticated packets as invalid. This authentication mismatch is a common exam trap because it is easy to overlook when basic IP connectivity and area configuration appear correct. In practice, routers will maintain Layer 3 connectivity but fail to exchange OSPF routing updates, causing routing issues. Understanding that OSPF adjacency depends on protocol-level agreement beyond IP reachability is critical for troubleshooting and exam success.
KKey Concepts to Remember
- OSPF routers must have matching authentication settings on their interfaces to successfully form neighbor adjacencies.
- OSPF adjacency formation requires agreement on area assignment, authentication, hello and dead intervals, and subnet mask consistency.
- Basic IP connectivity like successful ping does not guarantee OSPF neighbor formation if protocol parameters differ.
- OSPF authentication mismatches cause routers to discard OSPF packets, preventing neighbor relationships despite Layer 3 reachability.
- OSPF message-digest authentication (MD5) requires both routers to configure identical keys and authentication types on shared links.
- Router hostnames do not impact OSPF neighbor formation or adjacency establishment in any way.
- Subnet masks influence IP routing and reachability but do not block OSPF multicast traffic if correctly configured.
- OSPF neighbors must be in the same area and have matching interface parameters to exchange routing information and form adjacencies.
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.
Related practice questions
Related 200-301 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
CCNA subnetting practice questions
Practise IPv4 subnetting, CIDR, masks, host ranges and subnet selection.
CCNA OSPF practice questions
Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.
CCNA VLAN practice questions
Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
CCNA STP practice questions
Practise spanning tree, root bridge election, port roles and STP troubleshooting.
CCNA EtherChannel practice questions
Practise LACP, PAgP, port-channel behaviour and bundle requirements.
CCNA ACL practice questions
Practise standard and extended ACLs, permit/deny logic and traffic filtering.
CCNA NAT practice questions
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
CCNA DHCP practice questions
Practise DHCP scopes, relay, leases and troubleshooting.
CCNA show ip route practice questions
Practise routing-table output, longest-prefix match, AD and route selection.
CCNA show interfaces trunk practice questions
Practise trunk verification and VLAN forwarding across switches.
CCNA wireless security practice questions
Practise WLAN security, authentication and wireless architecture concepts.
CCNA IPv6 practice questions
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
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 settings on their interfaces to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: The OSPF authentication settings do not match. — The most likely cause is an OSPF authentication mismatch. In practical terms, the routers have basic IP connectivity and they agree on the area, so the failure is happening at the protocol trust level rather than at Layer 3 reachability. OSPF neighbors do not form just because they can ping each other. They also have to agree on parameters such as authentication, timers, area, and other interface-level settings. In this case, R1 is configured to use message-digest authentication, while R2 has no matching authentication configuration shown. That means R2 will not accept the authenticated OSPF packets in the expected way, and the neighbor relationship fails. This is a classic exam trap because the link itself still appears healthy.
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.
Discussion
Loading comments…
Sign in to join the discussion.