The second step in the link-state routing process is that each router is responsible for meeting its neighbors on directly connected networks.
Routers with link-state routing protocols use a Hello protocol to discover any neighbors on its links. A neighbor is any other router that is enabled with the same link-state routing protocol.
Click Play in the figure to view an animation on the link-state neighbor discovery process with Hello packets.
In the animation, R1 sends Hello packets out of its links (interfaces) to discover if there are any neighbors. R2, R3, and R4 reply to the Hello packet with their own Hello packets because these routers are configured with the same link-state routing protocol. There are no neighbors out the FastEthernet 0/0 interface. Because R1 does not receive a Hello on this interface, it does not continue with the link-state routing process steps for the FastEthernet 0/0 link.
When two link-state routers learn that they are neighbors, they form an adjacency. These small Hello packets continue to be exchanged between two adjacent neighbors and serves as a keepalive function to monitor the state of the neighbor. If a router stops receiving Hello packets from a neighbor, that neighbor is considered unreachable and the adjacency is broken.