The routing table and forwarding table contain the routes for the routers within a network. These routes are learned through the exchange of route advertisements. Route advertisements are exchanged according to the particular protocol being employed within the network.
Generally, a router transmits hello packets out each of its interfaces. Neighboring routers detect these packets and establish adjacencies with the router. The adjacencies are then shared with other neighboring routers, which allows the routers to build up the entire network topology in a topology database, as shown in Figure 47.
Figure 47: Route Advertisement
In Figure 47, Router A sends out hello packets to each of its neighbors. Routers B and C detect these packets and establish an adjacent relationship with Router A. Router B and C then share this information with their neighbors, Routers D and E, respectively. By sharing information throughout the network, the routers create a network topology, which they use to determine the paths to all possible destinations within the network. The routes are then distilled into the forwarding table of best routes according to the route selection criteria of the protocol in use.