4.4. Single Area OSPFv2

💡 First Principle: Unlike distance-vector protocols that rely on rumors ("my neighbor says that network is 3 hops away"), OSPF routers share facts—actual topology data. Each router has a complete map of the network and independently calculates shortest paths. This is why OSPF converges in seconds, not minutes, and why it doesn't suffer from routing loops: everyone has the same map, so everyone agrees on the best paths.

What happens without link-state visibility: Consider a network where Router A learns "network X is 3 hops away" from Router B. What if Router B's path goes through Router A? You've created a loop. Distance-vector protocols need hold-down timers and split-horizon to prevent this. OSPF doesn't have this problem—every router sees the complete picture and can detect loops before they happen.

Think of it like this: distance-vector is like asking for directions and being told "go north, then ask someone else." OSPF is like everyone sharing a complete road map—you can see every road and intersection and calculate the best route yourself.

Why OSPF dominates enterprise networks:

• Fast convergence: Network changes propagate in seconds
• No routing loops: Everyone has the same map, everyone calculates the same paths
• Scalable: Areas let you divide large networks into manageable pieces
• Open standard: Works across vendors (unlike EIGRP)

The CCNA focuses on single-area OSPF—all routers in Area 0, the backbone. Multi-area designs (Areas 1, 2, etc.) are CCNP territory, but you should know they exist.