Question 1mediummultiple choice
Full question →A network engineer enters the following commands on R1, but R1 does not form an OSPF adjacency with R2 on interface GigabitEthernet0/0.
What is the most likely cause of the failure?
mediummultiple choiceObjective-mapped
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.1.12.1 255.255.255.0 ip ospf hello-interval 10 ip ospf dead-interval 40 ! 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 ip ospf hello-interval 5 ip ospf dead-interval 20 ! router ospf 10 network 10.1.12.0 0.0.0.255 area 0
A network engineer enters the following commands on R1, but R1 does not form an OSPF adjacency with R2 on interface GigabitEthernet0/0.
What is the most likely cause of the failure?
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 would be a valid reason if one router were in area 0 and the other were in another area. In the exhibit, both routers place the same subnet into area 0, so the area setting is not the issue. The failure occurs at the interface parameter level instead.
B
Best answer
The OSPF timers on the interface do not match.
This is correct because the interfaces are configured with different hello and dead intervals. OSPF neighbors expect those timers to align, and if they do not, the routers reject the neighbor relationship. The addressing and area assignment are fine, but the timer mismatch blocks adjacency formation.
C
Distractor review
The subnet mask prevents multicast OSPF packets from being exchanged.
The subnet mask shown is a normal /24 LAN mask and does not stop OSPF multicast traffic. OSPF commonly uses multicast on broadcast segments without any problem in that mask size. The real incompatibility is the timer mismatch, not the mask.
D
Distractor review
The OSPF process IDs do not match on the two routers.
This sounds believable because many learners assume OSPF process IDs must match between neighbors. In reality, the process ID is locally significant and is not exchanged as a neighbor-forming parameter. Two routers can still become neighbors with different local process numbers if the real link parameters match.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is believing that OSPF process IDs must match between routers to form an adjacency. Many learners mistakenly think the process ID is a neighbor parameter, but it is actually locally significant and irrelevant to neighbor formation. Another trap is assuming that subnet mask differences or area mismatches are the cause when the real issue is timer mismatch. In this question, the hello and dead intervals differ between routers, causing adjacency failure. Misunderstanding these timer requirements leads to incorrect troubleshooting and exam answers.
Technical deep dive
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that requires routers to form neighbor adjacencies before exchanging routing information. For two routers to become OSPF neighbors, several interface parameters must match exactly, including the hello interval and dead interval timers. The hello interval defines how often a router sends hello packets to discover and maintain neighbor relationships, while the dead interval specifies the time a router waits without receiving a hello before declaring the neighbor down. OSPF adjacency formation follows strict rules: routers must be in the same subnet, configured with the same area ID, use compatible authentication settings, and have matching network types and timers on the interfaces involved. The hello and dead timers are critical because mismatched timers cause routers to perceive each other as incompatible, preventing adjacency formation. The OSPF process ID is locally significant and does not affect neighbor relationships. A common exam trap is assuming that OSPF process IDs must match between neighbors or that subnet masks alone can block adjacency. In practice, mismatched hello and dead intervals are a frequent cause of adjacency failure. Cisco routers will not form OSPF adjacencies if these timers differ, as the routers cannot synchronize their neighbor state machines. Understanding this behavior is essential for troubleshooting OSPF neighbor issues and ensuring stable routing in Cisco networks.
KKey Concepts to Remember
- OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
- The OSPF process ID is locally significant and does not need to match between routers for adjacency formation.
- OSPF adjacency requires routers to be in the same subnet and configured with the same area ID on the participating interfaces.
- Mismatched OSPF hello and dead timers cause routers to reject neighbor relationships, preventing adjacency formation.
- OSPF uses multicast hello packets sent at the hello interval to discover and maintain neighbor relationships.
- The dead interval timer defines how long a router waits without receiving a hello before declaring a neighbor down.
- Subnet masks do not block OSPF multicast packets if the routers are in the same subnet and area.
- OSPF adjacency formation depends on matching interface parameters including area, authentication, network type, and timers.
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 hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: The OSPF timers on the interface do not match. — In plain terms, the routers are speaking OSPF at different speeds, so they do not trust each other enough to become neighbors. OSPF requires several interface parameters to match before an adjacency can form, and the hello and dead timers are two of the important ones. On R1, the hello timer is 10 seconds and the dead timer is 40 seconds. On R2, the hello timer is 5 seconds and the dead timer is 20 seconds. Because those values are different, each router sees the other as incompatible. From a technical point of view, OSPF neighbors on the same link must agree on area, authentication, stub settings, network type in many cases, and the hello/dead intervals. The process ID is locally significant, so it does not need to match between routers. The addresses are in the same subnet and both interfaces are placed into area 0. That leaves the timer mismatch as the real cause of the failed adjacency.
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.