Chapter 2 Advanced Routing for FortiOS 5.0 : Open Shortest Path First (OSPF) : OSPF Background and concepts : How OSPF works : OSPF router discovery
  
OSPF router discovery
OSPF-enabled routers generate Link-State Advertisements (LSA) and send them to their neighbors whenever the status of a neighbor changes or a new neighbor comes online. As long as the OSPF network is stable, LSAs between OSPF neighbors do not occur. An LSA identifies the interfaces of all OSPF-enabled routers in an area, and provides information that enables OSPF-enabled routers to select the shortest path to a destination. All LSA exchanges between OSPF-enabled routers are authenticated.
When a network of OSPF routers comes online, the follow steps occur.
1. When OSPF routers come online, they send out Hello packets to find other OSPF routers on their network segment.
2. When they discover other routers on their network segment, generally they become adjacent. Adjacent routers can exchange routing updates. See “Adjacency”.
3. A DR and BDR are elected from the available routers using priority settings, and router ID. See “Designated router (DR) and backup router (BDR)”, and “DR and BDR election issues”.
4. Link state updates are sent between adjacent routers to map the topology of the OSPF area.
5. Once complete, the DR floods the network with the updates to ensure all OSPF routers in the area have the same OSPF route database. After the initial update, there are very few required updates if the network is stable.