Cisco Certified Network Associate (CCNA) Practice Exam

What does OSPF use to ensure that routers are part of the same subnet?

1. Access lists

2. Router IDs

3. Subnet masks

4. Authentication keys

The correct answer is: Subnet masks

In OSPF (Open Shortest Path First), subnet masks play a crucial role in ensuring that routers can determine if they are part of the same subnet, which is essential for establishing neighbor relationships. The subnet mask helps in identifying the network portion and the host portion of an IP address, allowing OSPF-enabled routers to ascertain if they can communicate directly with each other. When routers within the same broadcast domain or subnet exchange OSPF Hello packets, they will not form a neighbor relationship unless their subnet masks match, indicating that they are in the same IP subnet. This is necessary for the purpose of routing and ensuring efficient communication within a network. While the other options serve important functions in the OSPF protocol and networking in general, they do not directly contribute to determining whether routers belong to the same subnet as explicitly as subnet masks do. Access lists are utilized for filtering traffic or securing access to network segments, router IDs serve to uniquely identify each router in OSPF, and authentication keys are used to secure OSPF communication but do not assist in subnet identification. Therefore, the use of subnet masks is fundamental in OSPF for ensuring that routers are indeed part of the same subnet.