Because RIP processes routing information based solely on the receipt of routing table updates, it cannot ensure bidirectional connectivity. As Figure 52 shows, RIP networks are limited by their unidirectional connectivity.
Figure 52: Limitations of Unidirectional Connectivity
In Figure 52, Routers A and D flood their routing table information to Router B. Because the path to Router E has the fewest hops when routed through Router A, that route is imported into Router B's forwarding table. However, suppose that Router A can transmit traffic but is not receiving traffic from Router B due to an unavailable link or invalid routing policy. If the only route to Router E is through Router A, any traffic destined for Router A is lost, because bidirectional connectivity was never established.
OSPF establishes bidirectional connectivity with a three-way handshake. For more information, see Link-State Advertisements.