Cisco Certified Network Associate (CCNA) Practice Exam

Which of the following is NOT a requirement for OSPF routers to become neighbors?

1. Matching hello timer

2. Matching area ID

3. Different router IDs

4. Matching subnet mask

The correct answer is: Matching subnet mask

The correct response indicates that matching the subnet mask is not a requirement for OSPF routers to become neighbors. In OSPF (Open Shortest Path First), the establishment of adjacencies relies more on several key parameters that must align between routers. For OSPF neighbors to form, routers must have matching hello and dead intervals, a matching area ID, and they must be on the same network segment, or link layer broadcast domain (which indirectly relates to having matching subnet parameters). However, the requirement for different router IDs is not strictly essential for forming a neighbor relationship, although each router must have a unique ID to avoid conflicts in routing advertisements. In this scenario, the subnet mask does not directly influence the neighbor adjacency in the same way that the other attributes do. Routers can establish neighbor relationships even if they utilize different IP addresses as long as their configurations meet the essential criteria required by OSPF, such as area and hello timers. Hence, differing subnet masks do not prevent the formation of OSPF neighbor relationships as long as the basic networking conditions are satisfied.