Section 5.2: Steady-State Operation
Link-state protocols keep in touch with their neighbors by periodically exchanging small packets rather than complete routing updates. In OSPF, these packets are called Hello packets, which identify the subnet, the router sending the packets and a few other details. These Hello packets serve the same purpose as timed, regular full routing updates serve for distance vector protocols. When a router fails to hear Hellos from a neighbor for an interval called the dead interval, the router assumes that the silent router has failed. OSPF then marks the silent router as "down" in its topology database. The other router then runs the SPF algorithm to calculate new routes, based on the fact that one of the network's routers is now unavailable. In addition, the router that notices the failure immediately floods the new router or link status to its neighbors, with those routers forwarding the updated status to their neighbors, eventually flooding the new status information to all the routers in the network. This quick convergence of link-state protocols prevents the occurrence of loops.