Courseiva
← 200-301
Full test
hardmultiple choiceObjective-mapped

Exhibit

R1 G0/1: 10.1.12.1 255.255.255.0
R2 G0/1: 10.1.12.2 255.255.255.252
Both interfaces are up and in area 0

Exhibit: R1 is not forming an OSPF adjacency with R2 on GigabitEthernet0/1. Which mismatch below is the most likely cause?

Question 1hardmultiple choice
Full question →

Exhibit: R1 is not forming an OSPF adjacency with R2 on GigabitEthernet0/1. Which mismatch below is the most likely cause?

Full question →

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 routers are using different IP subnet masks on the shared link

That mismatch can stop OSPF adjacency on the segment.

B

Distractor review

OSPF cannot run on GigabitEthernet interfaces

It absolutely can.

C

Distractor review

Both routers are in area 0

That part is correct, not the problem.

D

Distractor review

The router IDs must match for adjacency to form

Router IDs must be unique, not identical.

Common exam trap

Common exam trap: answer the scenario, not the keyword

A common exam trap is confusing the need for matching router IDs with adjacency formation. While router IDs must be unique within an OSPF domain, they do not need to match for adjacency to form. Another trap is assuming OSPF cannot run on GigabitEthernet interfaces, which is incorrect because OSPF supports all standard interface types. The most tempting mistake is overlooking the subnet mask mismatch, which silently prevents adjacency by making routers believe they are on different networks, even if other parameters like area ID match. This subtle misconfiguration is often missed during troubleshooting.

Technical deep dive

How to think about this question

OSPF (Open Shortest Path First) is a link-state routing protocol that requires neighbors on a shared network segment to agree on several key parameters before forming an adjacency. One critical parameter is the IP subnet mask on the interface connecting the routers. Both routers must have interfaces configured within the same IP subnet and mask to recognize each other as neighbors and exchange OSPF Hello packets successfully. If the subnet masks differ, the routers perceive the link as belonging to different networks, preventing neighbor discovery and adjacency formation. The OSPF adjacency formation process involves routers exchanging Hello packets on interfaces configured for OSPF. These Hello packets include network information such as the subnet mask, area ID, and Hello/Dead intervals. If any of these parameters mismatch, including the subnet mask, the routers will not establish adjacency. Cisco IOS enforces this strictly because OSPF uses the subnet mask to determine the network boundary. Therefore, a subnet mask mismatch on a shared Ethernet segment is a common cause of adjacency failure. A frequent exam trap is assuming that router IDs must match for adjacency, which is incorrect since router IDs must be unique to identify routers distinctly. Another trap is thinking OSPF cannot run on GigabitEthernet interfaces, which is false as OSPF supports all common interface types. In practical networks, subnet mask mismatches often occur due to manual misconfiguration or inconsistent addressing schemes, leading to silent adjacency failures that can disrupt routing convergence and network stability.

KKey Concepts to Remember

  • OSPF routers must have matching IP subnet masks on shared interfaces to form neighbor adjacencies successfully.
  • OSPF Hello packets include subnet mask information that routers use to verify network consistency before adjacency formation.
  • A subnet mask mismatch causes routers to perceive different networks, preventing OSPF neighbor discovery on the shared link.
  • OSPF requires unique router IDs for each router; identical router IDs prevent proper OSPF operation but do not cause adjacency failure due to mask mismatch.
  • OSPF can run on GigabitEthernet interfaces without restriction, supporting high-speed Ethernet links in routing domains.
  • OSPF adjacency formation depends on matching parameters such as area ID, Hello/Dead intervals, and subnet masks on the shared segment.
  • A subnet mask mismatch is a common cause of OSPF adjacency failure on broadcast networks like Ethernet.
  • Routers in the same OSPF area with consistent subnet masks and interface configurations will successfully 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 IP subnet masks on shared interfaces to form neighbor adjacencies successfully.

What is the correct answer to this question?

The correct answer is: The routers are using different IP subnet masks on the shared link — For OSPF neighbors to form, key settings on a shared segment must match. A subnet mask mismatch on a broadcast network often prevents proper neighbor formation because the routers do not agree on the local network.

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.