Link State Protocol Concepts
There are two fundamental ways of propagating information through a network: Each router can advertise the destinations it can reach, either devices or networks, to each of its directly connected neighbors, or each router can advertise the state of each of its connections to other devices or to networks to every other router on the network.
When a router or intermediate system advertises a destination as reachable, it means that the router is either connected to the network, or that it knows a valid, loop-free path to reach the destination. How advertisements occur, and how advertisements are used, relate to the actual design of the routing protocol, its packet formats, and its algorithms.
Using Figure 1-9