To maintain routing information, OSPF routers complete the following generic link-state routing process to reach a state of convergence:
1. Establish Neighbor Adjacencies (Figure 1) - OSPF-enabled routers must recognize each other on the network before they can share information. An OSPF-enabled router sends Hello packets out all OSPF-enabled interfaces to determine if neighbors are present on those links. If a neighbor is present, the OSPF-enabled router attempts to establish a neighbor adjacency with that neighbor.
2. Exchange Link-State Advertisements (Figure 2) - After adjacencies are established, routers then exchange link-state advertisements (LSAs). LSAs contain the state and cost of each directly connected link. Routers flood their LSAs to adjacent neighbors. Adjacent neighbors receiving the LSA immediately flood the LSA to other directly connected neighbors, until all routers in the area have all LSAs.
3. Build the Topology Table (Figure 3) - After LSAs are received, OSPF-enabled routers build the topology table (LSDB) based on the received LSAs. This database eventually holds all the information about the topology of the network.
4. Execute the SPF Algorithm (Figures 4 and 5) - Routers then execute the SPF algorithm. The gears in the figure are used to indicate the execution of the SPF algorithm. The SPF algorithm creates the SPF tree.
The content of the R1 SPF tree is displayed in Figure 6.
From the SPF tree, the best paths are inserted into the routing table. Routing decisions are made based on the entries in the routing table.