OSPF routers send Hello packets on OSPF enabled interfaces. If all parameters in the OSPF Hello packets are agreed upon, the routers become neighbors. On multi-access networks, the routers elect a DR and BDR. On these networks other routers become adjacent to the DR.

Adjacent routers go through a sequence of states. Adjacent routers must be in the full state before routing tables are created and traffic routed. Each router sends link-state advertisements (LSA) in link-state update (LSU) packets. These LSAs describe all of the routers links. Each router that receives an LSA from its neighbor records the LSA in the link-state database. This process is repeated for all routers in the OSPF network.

When the databases are complete, each router uses the SPF algorithm to calculate a loop free logical topology to every known network. The shortest path with the lowest cost is used in building this topology, therefore the best route is selected.

Routing information is now maintained. When there is a change in a link state, routers use a flooding process to notify other routers on the network about the change. The Hello protocol dead interval provides a simple mechanism for determining that an adjacent neighbor is down. -

Interactive Media Activity Drag and Drop: OSPF State Flowchart When the student has completed this activity, the student will be able to identify the different OSPF neighbor states.

   

Web Links OSPF Packets http://www.juniper.net/techpubs/ software/junos50/ swconfig50-routing/ html/ospf-overview6.html