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DR and the next highest one becomes BDR. If priority number 0 is
specified for a router, it will not be eligible in the DR/BDR election.
Once the DR and BDR are selected, all the other routers within the
same OSPF area establish a relationship with them to further
exchange routing information. Any router switched on later will
accept the existing DR/BDR on the network regardless of its own
router priority. Since there is high demand on DR's central link-state
information control, Router Priority should be configured to elect
the most reliable router on a network as DR. BDR is elected at the
same time as DR, and has the same relationship establishment with
other routers in the area, in order to make the transition to a new DR
smoother if there is any failure of the primary DR.
In addition to relationship establishment, "Hello" packets also act as
Keepalive messages to keep track of the reachablility of the links.
The "Hello" packets are sent periodically with a predefined interval to
let routers know that other routers are still functional.
Update – "LSA"
Each router maintains "state" and "link" information for routing. A
"link" is an interface on the router and the "state" of the link is the
information including the interface address, network, neighbor
routers, and so on. These "link-state" information is stored in a
router's data structure called "link-state database".
When a router determines the DR by the "Hello" packet, it will
generate a link-state advertisement (LSA) and send it to DR. The DR
controls and updates its central link-state database and distribute the
database information to all the other routers in the same OSPF area.
After the initial exchange and the building of the database, each
router within the same OSPF area will contain an identical database,
which is a complete topology map of the area including the cost of the
links. Due to any change in routing information, a router will save a
new copy of link state into its database and send LSA to DR. The DR
then flood the update to all participating routers in the area to
synchronize the link-state database.
Path determination – "SPF"
D-Link Firewalls User's Guide
Chapter 10. Routing